Why am I getting a max message size exceeded error on some emails but not others?

Key findings

• Temporary provider issues: Mailbox providers may experience temporary glitches or configuration errors, leading to incorrect bounce messages for certain sends, even if the message size is within typical limits.
• Variable limits: Maximum message size limits can vary significantly between different email providers and even for different users within the same provider, potentially based on factors like account type or historical usage. For a detailed overview of these variations, consider reviewing email attachment and size limits explained by EmailLabs.
• Recipient mailbox quotas: Even if the message size is generally acceptable, an email may be rejected if the recipient's inbox is full or nearing its storage capacity. Some providers may return a size-related bounce in this scenario.
• Sender reputation influence: Some systems might apply more stringent size checks or a lower effective limit for senders with a questionable or developing domain reputation.
• Content overhead: Even without explicit attachments, heavily formatted emails with many images or complex HTML can unexpectedly push the message size beyond limits due to encoding overhead.

Key considerations

• Analyze bounce codes thoroughly: Look beyond the primary bounce message for additional details or secondary codes, which can sometimes reveal the true underlying issue, such as a mailbox being over quota.
• Monitor provider status pages: Check for known outages or temporary issues reported by the specific mailbox provider (e.g., Slingshot IT Support's advice on message size limits or similar support resources) that might explain inconsistent rejections.
• Optimize email content size: Reduce image sizes, avoid unnecessary HTML, and minimize attachments where possible. For insights into related issues, explore why unusually small email sizes might bounce.
• Understand recipient infrastructure: Be aware that smaller or older email providers might have more rigid or lower message size limits compared to larger ones.

Key opinions

• Inconsistent bounces: Marketers are puzzled when half of their emails to the same domain bounce for size issues, while the other half are successfully delivered, suggesting a non-uniform application of limits.
• Hard bounce confusion: The return of a "maximum message size exceeded" as a hard bounce, rather than a soft bounce, is unexpected and complicates standard bounce management procedures.
• Temporary blips: Some attribute such inconsistencies to specific, short-lived issues or connection problems experienced by the mailbox provider during a specific period.
• Mailbox full hypothesis: A common theory is that the size error might be a proxy for a recipient's mailbox being full, especially with smaller providers.
• Clear bounce messages: Marketers appreciate providers like Optonline that typically give straightforward and unambiguous bounce messages, even if the interpretation of a size error can still be complex. This is important for improving overall email deliverability.

Key considerations

• Validate content weight: Even without explicit attachments, marketers should check if their emails are too image-heavy or contain complex HTML structures that significantly increase message size.
• Leverage community resources: Consult forums and communities like Spiceworks for shared experiences and potential widespread issues with specific mailbox providers.
• Review email design for efficiency: Consider optimizing email design to be lighter, reducing overall byte size and minimizing the chance of hitting arbitrary limits.
• Distinguish bounce types: While some smaller providers may issue hard bounces for various reasons, it's important to differentiate between actual permanent failures and transient issues to manage lists effectively.

Key opinions

• Dynamic size limits: Experts suggest that message size limits are not always static and can vary based on a sender's IP or domain reputation. A lower reputation might lead to stricter size enforcement or reduced thresholds. This is critical for understanding how to recover domain reputation.
• Mailbox capacity: The message size error can often be a symptom of a recipient's mailbox being full, rather than the email itself being inherently too large for the provider's general limit.
• Attachments versus in-body content: Experts note that sending emails with attachments is inherently more challenging due to the added size and potential for complex encoding, which can trigger size limits more easily than in-body content.
• ISP-specific quirks: Some mailbox providers may have unique behaviors or temporary errors that cause mislabeled bounce codes, where a size error might actually represent another underlying issue.

Key considerations

• Proactive reputation management: Maintain a strong sender reputation to ensure that your emails are not subject to stricter size limits. Tools like Google Postmaster Tools can help.
• Content optimization: Even without explicit attachments, minimize the overall size of your email by optimizing images and simplifying HTML structure to reduce the likelihood of hitting hidden limits. Learn more about attachment and message size limits.
• Holistic troubleshooting: When facing inconsistent bounce messages, consider a range of potential causes beyond the literal error, including mailbox provider specific limits and recipient account status.
• Review bounce logging: Ensure your sending platform provides comprehensive bounce logging, including exact SMTP codes, to better diagnose the true underlying problem.

Key findings

• SMTP size negotiation: SMTP (Simple Mail Transfer Protocol) typically supports a SIZE extension that allows sending and receiving servers to negotiate the maximum acceptable message size before transmission begins. This negotiation can lead to variations in accepted sizes. For more on SMTP, explore what RFC 5322 says versus what actually works.
• Provider-specific limits: Despite a general SMTP standard, each mail server (like those for Outlook, Gmail, or smaller ISPs) implements its own specific maximum message size limits, which are often not publicly disclosed for all tiers or user types. These limits apply to the total encoded size of the email.
• Encoding overhead: When emails contain non-text elements (like images or attachments), they undergo encoding (e.g., Base64), which can increase the raw message size by approximately 33%. This overhead is often overlooked but contributes significantly to the final message size delivered to the server.
• Mailbox quota vs. message size: Documentation often clarifies that an email being too large for a mailbox (over quota) is distinct from it exceeding the server's maximum message size. However, some systems may return a general size error for both scenarios, creating ambiguity. More details on different common connection and message limits can be found.

Key considerations

• Total size calculation: When assessing email size, always account for the total encoded size, not just the raw size of attachments or text content. This includes HTML, CSS, images (encoded as part of the message or linked), and any attachments.
• Attachment best practices: For very large files, it is generally recommended by documentation to link to cloud storage or download pages rather than attaching the file directly to avoid hitting size limits and improving deliverability.
• Image optimization: Ensure all images are properly compressed and optimized for web use before embedding them in emails to minimize their contribution to the overall message size. This helps in avoiding common issues like Gmail clipping messages.
• Error code interpretation: Familiarize yourself with common SMTP bounce codes related to size and quota to better interpret the actual cause of rejection, as documented by various email service providers.