What does the email error '552 5.2.0 sender rejected AUP#POL' mean and how to resolve it?

Key findings

• Permanent rejection: The 552 SMTP code signifies a permanent failure, meaning the message will not be retried.
• AUP violation: AUP (Acceptable Use Policy) indicates that the sender's email content or behavior violates the recipient server's rules, often related to spam, phishing, or other abusive practices. POL likely stands for Policy.
• Sender-specific: The rejection is typically tied to the sender's specific IP address or domain, rather than the recipient address itself. Migrating email platforms often exposes new sending IPs to these policies.
• Cloudmark filtering: Many ISPs, including Cox and CenturyLink, use third-party spam filtering services like Cloudmark, which apply their own policies and blocklists (or blacklists).
• New domain/IP reputation: A common cause for this error, especially for new sending setups, is the lack of established reputation for the sender's IP address or domain. New IPs from providers favored by spammers can be particularly susceptible. Learn more about how long it takes to recover domain reputation.

Key considerations

• Check email authentication: Ensure your SPF, DKIM, and DMARC records are correctly configured for your new sending platform. Misconfigurations can severely impact your sender reputation. Our simple guide to DMARC, SPF, and DKIM can help.
• Warm up new IPs: If you've recently changed sending platforms or IP addresses, a proper IP warmup schedule is crucial to build a positive sending reputation with ISPs and avoid triggering AUP policies.
• Content review: Evaluate your email content for anything that might be flagged as suspicious, such as excessive links, image-to-text ratio, or spammy keywords. This also relates to why your emails go to spam.
• Contact the ISP: Although the error message may not contain direct contact details for Cox.com, contacting their postmaster or support team can provide specific insights into the AUP violation. This is similar to troubleshooting Cox.net rejections in general.
• Monitor blocklists: Regularly check if your sending IP or domain has been listed on any public blocklists (or blacklists). While Cloudmark often uses private blocklists, being listed elsewhere can contribute to overall reputation issues. Consult our guide to email blocklists.

Key opinions

• Migration impact: Marketers frequently see this error when switching email sending platforms, indicating that new sender IPs are not yet trusted, even if previous platforms didn't report issues.
• Generic error message: The error code AUP#POL (Acceptable Use Policy) is often too general to provide immediate actionable insights into the specific policy violation.
• Sender-specific behavior: The rejection is noted as being specific to the sender, implying that the issue lies with the sending domain's or IP's behavior, rather than the recipient address being invalid. This is similar to recipient address rejected errors.
• Widespread ISP impact: This error isn't limited to one ISP; marketers report seeing it across various regional ISPs, suggesting a common underlying filtering mechanism.
• Basic configuration checks: Initial troubleshooting often involves verifying fundamental setups like MX records and ensuring the sending address is legitimate, even if other mailbox providers are accepting mail without issues.

Key considerations

• Reach out to postmasters: Despite vague error messages, directly contacting the postmaster or support team of the affected ISP (e.g., Cox) is considered a primary step to get specific details on the policy violation.
• Understand the bounce: While it's a 552 hard bounce, the error doesn't always imply an invalid recipient, but rather a sender issue under current conditions.
• Identify filtering provider: Recognizing that major ISPs often use services like Cloudmark can direct marketers to the actual source of the filtering decisions, informing their next steps for resolution. This aligns with other sender rejected errors.
• Check email content: While not explicitly mentioned in the specific error, large attachments are a common cause for 552 errors, as discussed in community forums, suggesting that content size or type can trigger AUP violations.

Key opinions

• Cloudmark is key: Experts quickly identify Cloudmark as the underlying filtering service responsible for these rejections for ISPs like Cox and CenturyLink, based on MX record banners and IP ranges.
• PTR record issues: A primary cause identified is the age and reputation of the domain in the PTR record for the sending IP. New domains or those from certain hosting providers can trigger these policy violations. This is a common theme when your IP address is not authorized to send email.
• Not a recipient issue: Despite being a 552 permanent error, the consensus is that the problem isn't the recipient address itself but rather a condition related to the sender's compliance with policy.
• MX record integrity: Unreachable or misconfigured hosts within a sending domain's MX records, even at lower priorities, can cause issues with filters like Cloudmark that perform comprehensive checks.
• Check the banner: A crucial debugging step is to check the SMTP banner of the receiving mail server, as it often reveals the filtering service (e.g., Cloudmark) that is processing the connection.

Key considerations

• Full data sharing: Providing comprehensive data (IPs, domains, dates) to deliverability experts or filtering services like Cloudmark is essential for quicker resolutions.
• Contact Cloudmark directly: Given Cloudmark's involvement, reaching out to them is recommended if direct communication with the ISP doesn't yield results, as they have direct control over their filtering decisions. This can complement your blocklist monitoring.
• PTR record quality: Ensure your sending IPs have well-configured and reputable PTR records, as new or low-cost hosting IPs can be flagged by anti-spam systems.
• Domain reputation building: For new domains, focus on building a strong domain reputation over time through consistent, legitimate sending practices. For advice, see our guide on how to improve domain reputation.

Key findings

• Permanent error: RFC 5321 defines a 5xx SMTP reply as a permanent negative completion reply, meaning the mail transaction failed and the sender should not retry without modification.
• Mailbox capacity: The 552 error is typically associated with exceeding mailbox storage allocation or other data limits, though custom messages like AUP#POL expand its meaning to include policy violations. For example, SMTP Field Manual elaborates on this.
• AUP interpretation: Acceptable Use Policies (AUPs) are typically broad documents that define acceptable and prohibited behaviors for network users, including email senders. Violations can range from sending spam to exceeding size limits.
• Custom error codes: ISPs and email service providers (ESPs) often implement custom error codes and messages to provide more specific reasons for rejections, which may not be universally documented.

Key considerations

• Review ISP policies: When encountering AUP-related errors, consult the specific ISP's Acceptable Use Policy or email guidelines, such as Cox's email error codes, even if the exact sub-code isn't listed.
• Adhere to RFC standards: While custom codes exist, ensuring your email system adheres to fundamental SMTP RFCs (like RFC 5321 and RFC 5322) is critical for basic deliverability. Our guide What RFC 5322 Says vs. What Actually Works provides insights.
• Sender reputation scores: Underlying AUP rejections often relate to sender reputation scores maintained by ISPs or third-party filters, which are not always public. Consistent, good sending practices are key.