Getting an email delivered isn’t the same as getting it read — if it lands in spam, none of your content matters. This guide breaks down why emails miss the inbox (authentication gaps, poor list hygiene, low engagement) and how to fix it, covering the three levers that actually move deliverability: clean data, well-crafted creative, and smart program configuration (like mail stream separation and sending cadence).
It also looks at where the industry is headed — AI-driven personalization, BIMI branding, and stricter sender requirements from major inbox providers — plus real examples from brands like Amazon, Grammarly, Trello, and McDonald’s putting these practices to work. Whether you’re troubleshooting a sagging inbox rate or just want to future-proof your program, this is a practical playbook for making sure your emails actually get seen.
Introduction
Part 1: Brushing Up on the Basics
Part 2: Deliverability Landscape in 2024 and What to Look for in 2025
Part 3: Conquering Deliverability Challenges: A Threefold Strategy
Conclusion
Email marketing remains a highly effective channel for businesses, large and small, leveraging its capabilities for lead generation and revenue growth. According to a recent industry survey, email is one of the most effective marketing channels, with 79 percent of marketers placing it in their top 3. Additionally, the average return on investment (ROI) for email marketing is an impressive $36 for every $1 spent (Litmus).
Advancements in technology, particularly in artificial intelligence (AI), are further enhancing the effectiveness of email marketing. AI now helps marketers optimize send times, personalize content, and predict customer behaviors, which can significantly boost engagement rates. For instance, personalized emails are six times more likely to drive conversions compared to non-personalized ones (Porch Group Media). Furthermore, brands that frequently use dynamic content in their email campaigns report a higher ROI of 43:1 compared to those that seldom use such content (WPShout).
Email marketers face an uphill battle with deliverability. Despite tech advancements, getting emails to the right inbox remains a struggle. In 2024, nearly 1 in 6 U.S. sender emails missed the mark (EmailTooltester.com). Spam filters, sender reputation, and authentication all weigh in. Gmail and Yahoo have been and continue to be upping the ante with stricter spam thresholds and mandatory authentication (Google and Yahoo), pushing marketers to double down on deliverability best practices for campaign success.
Email deliverability can be challenging, but reliable strategies exist to improve outcomes. According to the Inboxable database, in Q2 of 2024 only 81.4% of emails, on average, successfully reached the inboxes of their intended recipients. Data Axle’s deliverability experts have drawn upon decades of experience working with enterprise-level marketers and internet service providers (ISPs) to craft this guide with insights into effective strategies to significantly boost email deliverability and enhance your campaign success.
To understand what deliverability is, it is important to understand the difference between email delivery rates and deliverability rates.
Delivery Email delivery rate represents the percentage of sent emails that were accepted by the recipient’s servers without bouncing. The general agreed-upon average bounce rate is below 2%, which means more than 98% of emails are delivered. However, this metric fails to consider the placement of the delivered email. Just because it is delivered does not mean it reaches the inbox. On a side note, we recommend keeping the bounce rate below 1%.
Deliverability Deliverability, on the other hand, takes placement into consideration, and it is a more accurate assessment of whether the email reaches the inbox. It is measured by using:
As of 2025, individuals and businesses send and receive approximately 376.4 billion emails each day, marking a significant increase from the 306.4 billion daily emails recorded in 2020. How did those emails make it to the inbox? To put it simply, the process looks like this:
An email sent to someone who has not given permission to be contacted (i.e., hasn’t opted into your subscriber list) is spam.
Spam folder placement occurs when an email message is flagged by spam filters and diverted to the recipient’s junk or spam folder instead of landing in the inbox. This can happen for various reasons, such as suspicious content, poor sender reputation, or inadequate email authentication.
When emails are bulked, they are less likely to be seen or opened by recipients, significantly reducing the effectiveness of your email campaigns. Therefore, it’s crucial to monitor and address bulking issues promptly to maintain optimal deliverability and ensure higher engagement with your audience.
As a form of “checks and balances” and to ensure that users do not receive unauthorized emails, ISPs have set up spam traps as a way to identify senders who email without permission. There are three types of spam traps used to identify spammers:
In recent years, the email landscape has become dominated by a few large internet service providers, creating an almost monopolistic environment. The most prominent email service providers include Google (Gmail), Microsoft (Outlook/Hotmail), Yahoo (Yahoo Mail), and Apple (iCloud Mail). Since its launch in 2004, Gmail has become the most popular email service provider, with over 1.8 billion users (as of Jan 2025). Microsoft Outlook follows with 400 million users, and Yahoo mail falls into third place with 227.8 million users. AOL holds a smaller portion of the pie with 1.5 million users.
The concentration of power among a limited number of internet service providers (ISPs) can have profound implications for email senders. ISPs hold the keys to email deliverability, wielding the ability to filter and prioritize emails based on sender reputation. This reputation is influenced by various factors, including email engagement rates, content quality, and adherence to authentication protocols. A positive reputation can lead to higher deliverability rates, while a negative one can result in emails being relegated to spam folders or blocked entirely. Shared IP addresses compound the issue, as the behavior of all users on that IP can affect individual reputations. To navigate this landscape, we have broken down ISPs’ best practices to ensure deliverability.
Authentication
Authentication protocols such as SPF (sender policy framework), DKIM (DomainKeys identified mail), and DMARC (domain-based message authentication, reporting & conformance) play a crucial role in helping ISPs identify genuine senders and categorize emails as safe. These protocols are essential for distinguishing legitimate emails from spoofed or forged messages often used in phishing scams. Emails lacking proper authentication are typically diverted to spam folders or rejected altogether by ISPs.
Blocklist
ISPs maintain blocklists (formerly known as blacklists), which are real-time databases of IP addresses known for sending spam. Being listed on a blocklist can lead to ISPs blocking your email campaigns. Factors contributing to blocklisting include poor list quality, sudden spikes in sending volume, or recipient complaints. ISPs may refer to their own blocklists or consult public databases like Spamhaus and Barracuda when deciding whether to block emails from a particular sender.
Encryption
Encryption, particularly transport layer security (TLS) encryption, is crucial for securing email communications between senders and recipients. Messages lacking TLS encryption are flagged by ISPs, reducing their chances of reaching the inbox. Even if they do reach the inbox, unencrypted emails may display a red open padlock icon, signaling potential security risks to recipients and diminishing engagement likelihood.
Sender reputation management
ISPs utilize sophisticated algorithms to determine whether your email lands within the inbox or the junk folder. Many factors influence this decision, with each score being tied to the sender’s IP address and domains. This score reflects your reputation based on factors such as user engagement, adherence to email best practices, sending volumes, the implementation of authentication, and encryption measures. Should your sender score dip below a predefined threshold, ISPs may reroute your emails to the spam folder rather than the inbox.
These metrics inform ISPs how their users interact with message from specific senders and, in turn, determine the rate at which a message will inbox.
Over the years, the evolution of email has seen one (ISP) rise to prominence: Gmail. With a significant global user base of 1.8 billion, Gmail is a critical platform for email marketers. Its substantial share of the email client market makes it a primary focus for businesses striving to connect with their audiences effectively.
However, the challenge of achieving high deliverability to Gmail inboxes is notable, particularly as Gmail employs sophisticated algorithms to filter emails. On average, only 67.9% of emails reach Gmail inboxes, which is significantly lower than the overall average inboxing rate of 81%.
Several factors contribute to the low deliverability rate of Gmail. First, Gmail has sophisticated spam filters that evaluate various elements, including email content, sender reputation, and user feedback. Emails containing certain keywords, phrases, or images may trigger these spam filters. Additionally, poorly formatted HTML or a high image-to-text ratio can also cause an email to be flagged as spam.
Beyond the high spam rate, Gmail’s complex interface adds to the challenge. Even when emails are successfully delivered, they don’t always land in the primary inbox. They might appear in tabs like Promotions or Social—places where engaged users often look for brand updates and deals. While this can shift how messages are seen, it still offers valuable visibility to an audience actively seeking marketing content.
To enhance deliverability, it is essential to adhere to best practices such as maintaining a low spam complaint rate, ensuring content relevance, and complying with Gmail’s authentication requirements. Given the lower percentage of emails that reach Gmail inboxes, it becomes even more crucial to optimize email campaigns for this platform.
Strategies such as segmenting your audience, personalizing content, and regularly cleaning your email list can greatly enhance the chances of your emails reaching the inbox, thereby boosting visibility and engagement with Gmail users.
The email deliverability landscape in 2025 will continue to evolve, driven by advancing technologies, stricter security protocols, and rising consumer expectations. To stay effective, organizations must navigate these changes and refine their email marketing and communication strategies. These developments bring unique challenges for both experienced marketers and those new to the field. This section will explore the key deliverability trends and shifts for the year ahead.
In 2024, Yahoo and Gmail introduced stricter email-sending standards to improve security and reduce spam. These changes require senders to authenticate their emails using established protocols like SPF, DKIM, and DMARC; provide a one-click unsubscribe option for recipients; and maintain a low spam complaint rate—ideally below 0.3%, with some providers recommending even stricter thresholds for optimal reputation. These measures aim to enhance trust, improve user experience, and ensure that legitimate emails reach inboxes.
Authentication methods, such as SPF, DKIM and DMARC are required for mass senders. These protocols ensure that emails are genuinely from the claimed sender, enhancing trust with both recipients and email service providers.
Spam rate
The main takeaway for mass email senders who send more than 5,000 daily emails to Google and Yahoo addresses is that you must authenticate your emails, implement one-click list-unsubscribe, and maintain a spam complaint rate under 0.3%, which means no more than three spam reports for every 1,000 emails. Although Gmail’s official acceptable spam rate is 0.3% or below, Google also recommends that senders keep their daily spam rates under 0.1% to maintain a good reputation.
One-click unsubscribe
Implementing a one-click unsubscribe feature is crucial for complying with anti-spam regulations and enhancing the user experience. This feature allows recipients to easily opt out of future emails with a single click, reducing spam complaints and maintaining a clean and engaged mailing list. By adhering to these requirements, senders can foster better relationships with their audience and ensure that their emails reach the intended inboxes.
Looking ahead to 2025, we can expect other email providers to adopt similar standards, further aligning the industry around authentication and complaint management. Innovations in authentication, such as automated tools for configuring SPF, DKIM, and DMARC, may emerge to simplify compliance. Additionally, the push for enhanced user control could lead to more intuitive unsubscribe options and transparency in email practices. Organizations that proactively adapt to these evolving standards will likely see stronger deliverability rates and improved audience trust.
In 2024, AMP (Accelerated Mobile Pages) emails have emerged as a growing trend. These interactive and dynamic emails allow recipients to engage directly with content without needing to open a separate web page. Developed by Google, AMP technology enhances traditional email by enabling interactive elements such as carousels, accordions, forms, and more. Looking ahead to 2025, we predict this trend will continue to grow, further enhancing email interactivity and user engagement.
AMP emails offer a range of interactive features, allowing users to engage with elements like forms, event RSVPs, and product carousels directly within their inboxes. They also provide real-time updates, ensuring that recipients always see the latest information, such as live inventory status or current pricing. This interactive and dynamic nature enhances user engagement and can lead to higher conversion rates. Additionally, AMP emails support personalized and targeted messaging based on user behavior and preferences, creating a more tailored and effective communication experience.
While AMP emails offer many benefits, they also come with certain drawbacks. Implementing AMP emails can be complex and challenging to learn, requiring a solid understanding of AMP specifications and careful validation of the content. The learning curve can be steep for email marketers who are not already familiar with AMP technology. Additionally, sending AMP emails necessitates precise navigation to maintain a good sender reputation. Errors in implementation or failure to comply with AMP standards can lead to deliverability issues or even being flagged as spam. Therefore, while AMP emails can enhance engagement and interactivity, marketers must invest significant effort and attention to detail to use them effectively and maintain their reputation with email clients.
In 2024, BIMI (Brand Indicators for Message Identification) continued to gain traction as a critical component of email authentication and brand recognition strategies. Major email providers expanded their support for BIMI, making it more accessible to businesses of all sizes. The introduction of simplified certification processes and broader adoption of Verified Mark Certificates (VMCs) helped lower the barrier to entry, enabling more brands to display their logos in inboxes.
This shift not only enhanced trust but also improved engagement rates as recipients grew more accustomed to identifying authentic emails at a glance. Furthermore, advancements in BIMI-related analytics provided deeper insights into the impact of brand logos on email performance, helping organizations fine-tune their strategies.
Looking ahead to 2025, BIMI is expected to evolve further with increased standardization and global adoption. As more email clients join the ecosystem, interoperability will improve, making it easier for brands to implement BIMI across different platforms. Innovations in logo design and dynamic branding are anticipated, allowing businesses to personalize their visual identity in real time based on recipient demographics or campaign goals.
Additionally, regulatory bodies and industry groups may introduce new guidelines to ensure that BIMI remains secure and trustworthy, addressing potential misuse or fraudulent attempts. These developments position BIMI as a cornerstone of email deliverability and brand visibility in the years to come.
Like many other industries, AI has significantly disrupted email marketing and deliverability by introducing advanced personalization, segmentation, and automation capabilities. Among all functions, marketers lead the way in adopting generative AI. According to recent surveys, 65% of organizations are regularly using generative AI (gen AI), which is nearly double the percentage from just 10 months ago. In fact, 34% of respondents in the survey, specifically from the Marketing and Sales sector, frequently use generative AI (McKinsey), making it the highest percentage across sectors. Gen AI can be a wonderful tool for personalized content targeting different segments.
In addition, AI algorithms can analyze vast amounts of data to identify patterns in customer behavior, enabling marketers to tailor their email campaigns more effectively to individual preferences. This personalization extends to the timing, content, and subject lines of emails, which can be dynamically adjusted to increase engagement rates and avoid spam filters. AI-driven tools also offer predictive analytics, forecasting the performance of email campaigns and suggesting improvements.
More importantly, AI can contribute to data hygiene by cleaning up email lists to improve deliverability, ensuring that emails reach the intended inboxes. By automating routine tasks, AI allows marketers to focus on strategy and creative aspects, leading to more efficient and effective email marketing efforts.
However, the rapid adaptation of AI might lead to unexpected consequences. For example, the unsubscribe rate might become less relevant. As filters improve in automatically predicting, identifying, and removing non-engagers, the need for manual unsubscribes could diminish. Therefore, it is important to reassess metrics and KPIs for email campaigns where AI was employed.
Deliverability challenges typically arise from three key factors: poor data quality, ineffective creative elements (including email content and design), and improper email program configuration. To ensure that your emails consistently reach the inbox, focus on three critical areas: maintaining clean data and mailing lists, crafting compelling and personalized messaging, and optimizing program mechanics (such as mail stream separation, volume, and frequency).
Marketers are constantly focused on expanding and engaging their subscriber base, recognizing its pivotal role in revenue generation. After all, subscribers represent the lifeline of conversion, transitioning from email openers to clickers, website visitors, and ultimately, purchasers. However, the menace of poor customer data looms large—it’s capable of landing your business on blocklists and causing ISPs to intercept your emails. Achieving revenue growth and fostering customer relationships become daunting tasks when your emails fail to land in the inbox. Implementing the following data best practices can enhance your inboxing rate:
The best way to ensure that subscribers have knowingly opted into your mailing program is to validate each new email address when it is first provided, be it via a landing page, in person at a brick-and-mortar location, or through a referral. Verifying email addresses can be done in a few different ways:
By ensuring that your database remains clean and implementing these practices, you also protect against list bombing. List bombing is a malicious tactic involving automated software that floods a brand’s database with fake, inactive, or unauthorized email addresses. This not only compromises data integrity but can also damage sender reputation and lead to deliverability issues. Staying vigilant against such threats is essential for maintaining the security and effectiveness of your email marketing efforts.
Zero-party data refers to information that your audience shares directly and proactively about their preferences and interests, such as through surveys, polls, or preference centers. By leveraging preference centers, you can gather essential insights into how your audience wants to engage with your brand. Empowering your audience to control their relationship with your brand can lead to higher engagement with your messages, help you determine the optimal sending frequency, and minimize the risk of emails being marked as spam—all of which positively impacts deliverability. Through preference centers, subscribers can share their preferences on:
Another advantage of email preference centers is their role in keeping your subscribers happy and minimizing unsubscribes. ISPs have simplified the unsubscribe process by adding buttons to the top of senders’ email campaigns or suggesting which promotional emails to unsubscribe from based on engagement.
Gmail introduced a one-click unsubscribe option, providing end users with a quick and convenient way to opt out of emails. However, the traditional unsubscribe link within the message—ideally placed prominently, such as at the top of the email—can still serve as a gateway to a preference center. This allows users to “opt down” by reducing email frequency or updating their preferences, rather than fully unsubscribing from communications.
Over time, subscriber lists naturally accumulate invalid, inactive, or outdated email addresses, which can negatively impact email performance and sender reputation. Without regular data hygiene, these issues can lead to increased bounce rates, reduced engagement, and even ISP blocks. Prioritizing regular data hygiene practices is essential to safeguard your sender reputation, as sending emails to invalid or disengaged addresses signals to ISPs that you’re not maintaining a quality list, potentially harming your reputation and email placement.
Maintaining a clean list also improves deliverability by reducing hard bounces and spam complaints, ensuring that your messages are more likely to land in the inbox rather than the spam folder. Additionally, it boosts engagement rates by targeting active, interested subscribers, leading to higher open and click-through rates. With a healthy list, you can optimize campaign performance by focusing on creating content for an audience that genuinely values your communications, which maximizes your return on investment.
Finally, regular data hygiene helps reduce costs, as many email service providers charge based on list size. By removing non-responsive or invalid contacts, you can avoid unnecessary expenses and allocate resources more effectively.
These practices ensure that your email campaigns remain effective, relevant, and aligned with best practices for long-term email marketing success.
You can achieve this by:
While open and click rates provide valuable insights, they offer only a partial view of subscriber interaction. With advanced analytics, you can gain a deeper understanding of your subscribers’ behavior and preferences. For example, you can uncover the duration of subscriber engagement with your emails, revealing how long they stay engaged with your content. Additionally, you can learn which platforms and devices your subscribers prefer for email interactions, allowing you to optimize your strategy for better engagement and performance.
By leveraging these additional data points, you can uncover valuable insights, leading to more meaningful engagement and lasting relationships.
Your email’s design and content play a crucial role in whether it lands in the inbox or gets filtered out by ISPs based on engagement data. As a marketer, you have the power to enhance your inboxing rates by implementing effective design and content strategies. Consider incorporating the following tactics into your email campaigns to improve engagement:
A study from Litmus revealed that 43% of Gmail users read their emails without images, and both Yahoo and Outlook’s desktop applications have images turned off by default. While these emails may look great in the design phase, they often perform poorly in real-world scenarios where a significant portion of the audience either has images turned off or faces delays in loading them.
This challenge is further compounded for AI email summarization tools, which rely heavily on text-based content to generate accurate and meaningful summaries. Image-only templates lack the necessary textual elements for these tools to interpret, making it difficult to summarize or extract key insights. When recipients can’t immediately access your copy or CTAs due to blocked images, they disengage, negatively impacting inboxing rates, click-through rates, and revenue per email.
Brands can mitigate these issues by incorporating live text alongside images in their emails, ensuring accessibility and usability for both human readers and AI tools.
According to Statista, nearly 55% of global website traffic is generated from mobile devices, excluding tablets. Forty-one percent of email views come from mobile devices, while 39% come from desktop computers. The remaining 20% of email views come from tablets and other devices (Mailbutler).
Yet while 56% of marketers leverage mobile-friendly emails in their email marketing strategy, according to HubSpot Research, only 35% of email marketers say they use a mobile-first or mobile-responsive design process (Litmus). This presents a significant challenge for brands. Weak mobile email design can result in nearly half of your email opens failing to convert into clicks and purchases, leading to declining engagement and increased unsubscribes, ultimately impacting deliverability. To address these challenges, it’s crucial to prioritize responsive design.
With the ability to preview messages across various devices before deployment, you can identify and address any issues proactively, ensuring that your emails and CTAs are easily readable and clickable across all platforms.
To see the impact of responsive design in action, consider Breakfast with Audrey, an online magazine that blends timeless content with a modern digital strategy. Its email campaigns are structured around a single-column layout, optimized for easy scrolling and high readability on mobile devices. Key elements—such as imagery, call-to-action (CTA) buttons, headlines, and social links—are spaced generously and sized appropriately for smaller screens. The result is an email experience that not only looks polished but also performs, driving higher click-through rates by reducing cognitive load and friction.
Another standout is the Swedish jewelry brand Caroline Svedbom. Its mobile-optimized emails feature consistent CTAs, strong visual hierarchy, and responsive product images that maintain their integrity across screen sizes. The design highlights both the product and its packaging in a way that feels curated for the user’s device. By taking a mobile-first approach, the brand has created a seamless user experience that boosts engagement and supports conversion across platforms.
Grammarly’s weekly email campaign demonstrates best practices in subject line clarity and trust-building language. • Subject Line: Your Weekly Writing Update • It avoids promotional buzzwords, instead offering a value-based, informative subject line that personalizes the experience. • The content inside uses natural language and relevant tips, steering clear of aggressive sales copy. Link with Caution: When including links in your emails: • Maintain a balanced text-to-link ratio; avoid overloading with links. • Refrain from displaying lengthy or full URL links. • Avoid using generic link shorteners. • Ensure that your links lead to reputable and secure websites. In Grammarly’s emails, hyperlinks are embedded within meaningful anchor text like “View your stats” or “Improve this sentence.” There are no long URLs or questionable redirects—only clean, branded links that enhance deliverability and user confidence.
Craft Compelling CTAs: Engaging calls-to-action can significantly impact your inboxing rate, as they contribute to your sender reputation. ISPs consider metrics like unique click-through-rate and click-to-open rate when assessing your emails. Follow these CTA best practices: • Experiment with creative and attention-grabbing copy for your CTAs. • Utilize action-oriented language, such as “book now” or “get started.” • Employ design elements to draw subscribers’ attention to your CTAs. • Avoid overwhelming your audience with too many CTAs, which can diminish engagement.
Trello’s product update email is a model of clean CTA strategy: • Subject Line: New Features to Boost Your Productivity • Uses verbs like “boost” without falling into spam-trigger language. • Inside the email, CTAs like “Unlock Greatness” and “Turn the Page” are clearly styled and spaced, with a single, focused action per section. • All links route to secure, brand-consistent pages.