Is my email being blocked due to an acceptable use policy or content?
Michael Ko
Co-founder & CEO, Suped
Published 24 Jul 2025
Updated 18 Aug 2025
6 min read
Sending emails can sometimes feel like navigating a minefield, especially when your messages are unexpectedly blocked. You might have a great sender reputation, but suddenly some emails aren't reaching their intended recipients. A common culprit behind these deliverability issues is a violation of an acceptable use policy (AUP) or specific content triggering filters. These policies are in place to prevent spam, phishing, and other malicious activities, but they can sometimes inadvertently catch legitimate emails.
An AUP defines what is considered appropriate use of an email service. When an email service provider or internet service provider (ISP) detects content or sending behavior that violates their AUP, they may block your emails. This can manifest as an SMTP error, often indicating a policy violation or content issue. Understanding these policies and how your email content is perceived is key to ensuring your messages land in the inbox.
An acceptable use policy (AUP) is a set of rules applied by an email service provider or ISP that governs how their services can be used. These policies are designed to protect their network, users, and overall ecosystem from abuse. Violations often include sending unsolicited bulk email (spam), distributing malware, phishing, or sending content deemed illegal, offensive, or harmful. If your email traffic is inconsistent or includes certain patterns, it could be flagged.
Many providers, like Mailgun and Lumen, clearly outline their AUPs. They often cover everything from email volume and frequency to the types of content permitted. A common error message you might encounter that directly points to an AUP violation is '552 5.2.0 sender rejected AUP#POL'. If you see this, it indicates a strict policy enforcement, and you should investigate the specific cause. We have a dedicated guide on what that error means and how to resolve it.
Common AUP violations
Unsolicited bulk email: Sending large volumes of emails to recipients who have not explicitly opted in.
Content deemed illegal or harmful: This includes phishing attempts, malware distribution, or deceptive advertising.
Deceptive practices: Using misleading subject lines, 'from' addresses, or hidden content.
Exceeding sending limits: Sending more emails than your service provider allows within a given timeframe.
Even legitimate businesses can inadvertently violate AUPs. For instance, an outdated email list or a sudden spike in sending volume for a marketing campaign might trigger automated defenses. ISPs (internet service providers) and email providers use sophisticated algorithms to detect these violations, often without human intervention, leading to immediate blocking of your mail.
How content triggers blocking
Beyond explicit AUP violations, the actual content of your email can directly lead to it being blocklisted or flagged as spam. Spam filters analyze various elements within your message. This includes specific keywords, suspicious link patterns, certain types of attachments, and even the formatting of your email. Providers like Google outline sender guidelines that often touch upon content best practices to avoid filtering.
Common content triggers include excessive use of spammy phrases ('earn money fast', 'free offer'), suspicious URLs, or images with disproportionate text. Filters also look for hidden content, which can be a strong indicator of spam. If your email contains third-party advertisements, it's particularly important to ensure their content aligns with best practices to prevent getting spam blocked.
Content pitfalls
Spammy keywords: Words and phrases commonly associated with unsolicited email.
Broken or suspicious links: URLs that are shortened, irrelevant, or point to disreputable sites.
Excessive capitalization or exclamation marks: Used to create urgency or exaggerate claims.
Image-only emails: Emails primarily composed of images with little text can trigger filters.
Best practices for content
Clear and concise subject lines: Accurately reflect the email's content without hype.
Reputable links: Ensure all URLs in your email lead to trusted and relevant destinations.
Balanced text-to-image ratio: Include sufficient text to provide context for images.
Personalization: Tailor content to individual recipients where appropriate.
The consistency of your mailing history also plays a role. If your emails typically deliver successfully with occasional failures, it's a strong indicator that specific content in those failing messages is causing the issue. These intermittent blocks are harder to diagnose than consistent ones, as they suggest that the filter is reacting to particular elements that are not present in every email.
Technical factors impacting content delivery
While AUPs and content are direct causes of blocking, underlying technical configurations can amplify these issues. Email authentication protocols such as SPF, DKIM, and DMARC play a crucial role. If these are not correctly set up, even legitimate content can be flagged as suspicious. Unauthenticated emails are more likely to be blocked by major providers like Gmail.
Your sending IP address and domain reputation are also key. If your IP or domain is listed on a public or private blocklist, your emails may be blocked regardless of content. Sometimes, email addresses or domains advertised within your email content can also be blocklisted, which can lead to blocks even if your sending domain is clean. Regularly checking blocklist statuses is a proactive measure.
Factor
Description
Impact on deliverability
Acceptable Use Policy (AUP) Violation
Sending behavior or content that violates the ISP's terms.
Direct blocking, often with an 'AUP' error code.
Spam Content Keywords
Presence of words/phrases commonly found in spam.
Emails routed to spam folders or rejected outright.
Suspicious URLs/Links
Links to known bad sites or highly shortened URLs.
Increased spam score, potential blocking.
Lack of authentication (SPF/DKIM/DMARC)
Missing or misconfigured email authentication records.
Major email providers may reject or quarantine messages.
Troubleshooting and preventing content-based blocks
If your emails are being blocked due to content or an AUP, a systematic approach is necessary. First, review the bounce messages carefully. They often contain error codes or descriptions that provide clues, such as '552 5.2.0 sender rejected AUP#POL'. Next, analyze the content of the blocked emails. Look for common spam triggers, unusual formatting, or questionable links.
Regularly monitor your email deliverability rates and sender reputation using tools like Google Postmaster Tools. Pay attention to spam complaint rates and delivery errors. If your domain reputation is low, it can lead to more aggressive filtering. You can learn how to improve your domain reputation for long-term success.
Example: Analyzing a potential content issuetext
Subject: "Exclusive Offer! Get Rich Now!"
Body: "Click here for a FREE financial guide! Limited time offer!"
Links: http://tinyurl.com/xyz123 (redirects to a dubious site)
Issues: Spammy keywords, excessive urgency, suspicious shortened URL.
To prevent future blocks, establish clear internal content guidelines for your email campaigns. Train your marketing and sales teams on what content is acceptable and what could trigger filters. Always test your emails before sending large volumes, especially if you've made significant changes to your content or sending patterns. Maintaining a clean and engaged subscriber list also reduces the risk of spam complaints, which can severely impact your reputation.
Final thoughts
Email deliverability is a dynamic field, constantly evolving with new filtering technologies and policy updates. Being proactive about understanding acceptable use policies and meticulously reviewing your email content are essential steps for reliable inbox placement. It's not just about avoiding blocklists, but also about building trust with your recipients and email providers. When emails are blocked by a recipient's server due to content or policy, it can significantly hinder your communication efforts.
Staying informed about why your emails might be failing and continuously optimizing your sending practices will safeguard your email program. Even if your sender reputation is good, a single policy violation or problematic content element can cause delivery issues.
Views from the trenches
Best practices
Implement robust email authentication (SPF, DKIM, DMARC) to build trust with ISPs and reduce the likelihood of content-based blocking.
Regularly audit your email content for spammy keywords, suspicious links, and deceptive formatting. Use a content checker if available.
Segment your email lists and personalize content to reduce generic messaging that might trigger spam filters.
Monitor email bounce codes for specific error messages like 'AUP#POL' to pinpoint policy violations.
If using third-party services for links or tracking, ensure their reputation is solid to avoid indirect content flags.
Common pitfalls
Ignoring intermittent email blocks, assuming they are isolated incidents, when they might indicate a recurring content issue.
Not reviewing the AUP of your email service provider or the recipient's ISP, leading to unintentional policy violations.
Using generic or copied content across campaigns that might have been previously flagged as spam.
Including too many short, untrusted, or irrelevant links in your email body.
Failing to maintain a clean and engaged subscriber list, leading to higher spam complaints.
Expert tips
Always include an easy-to-find unsubscribe link in marketing and bulk emails.
Test email content variations with an email deliverability tester to predict potential blocking issues.
Keep subject lines clear and honest, avoiding misleading or overly promotional language.
Ensure a good text-to-image ratio in your emails, with proper alt text for images.
Regularly warm up new IPs or domains to build a positive sending reputation.
Expert view
Expert from Email Geeks says that an intermittent blocking issue often points to content-related problems within specific messages rather than overall reputation.
January 28, 2019 - Email Geeks
Marketer view
Marketer from Email Geeks says that seeing "AUP" in a bounce message likely means an acceptable use policy violation, which is directly tied to the email's content or sending behavior.
Google Postmaster Tools. Pay attention to spam complaint rates and delivery errors. If your domain reputation is low, it can lead to more aggressive filtering. You can learn how to improve your domain reputation for long-term success.
