Summary
The 'max message size exceeded' error occurs intermittently due to a combination of factors. Mail servers, including sender, recipient, and intermediary servers, enforce varying size limits. These limits may be affected by the sender's IP/domain reputation. Some servers have historical limitations, while others have specific limits like Gmail's 25MB. Encoding (e.g., base64), headers, and attachments all contribute to the overall size. Additionally, recipient mailbox limits can cause rejections. Some email providers have also had known issues relating to size limits. In summary, some emails exceed a size limit imposed at some point in the delivery chain, even if other emails are delivered successfully.
Key findings
- Varying Limits: Different mail servers have different, and sometimes undocumented, message size limits.
- Reputation Influence: IP/Domain reputation can affect message size limits.
- Encoding Impact: Encoding methods like Base64 increase the actual message size.
- Mailbox Capacity: Recipient mailbox capacity can trigger size-related bounce messages.
- Provider Issues: Known issues exist where some providers incorrectly reject messages.
- Server settings: Email size limits are also related to server settings and configurations. Not all servers are the same and might have different rules related to message size limits. And a message can pass through various servers before the final delivery, and each has its rules.
Key considerations
- Optimize Size: Minimize attachments (use links), compress images, and optimize headers to reduce email size.
- Check Limits: Check the sending server’s size limit and be aware of common recipient server limits (e.g., Gmail 25MB).
- Monitor Reputation: Maintain a good sending reputation to avoid stricter size limits.
- Test Deliverability: Regularly test deliverability, including sending test messages of varying sizes.
- Encoding impact: Consider the impact of encoding on the final message size.
- Be aware of ISPs: Be aware of any known size limitations of specific ISPs your recipients use.
What email marketers say
10 marketer opinions
The 'max message size exceeded' error occurs when an email exceeds the size limit imposed by one or more mail servers involved in the sending or receiving process. Different servers, including the sender's, recipient's, and intermediary servers, may have varying size limits. Factors such as attachments, encoding, headers, and server configurations can affect the overall size of an email. There can be a discrepancy between stated and actual limits, and some ISPs may have low limits, increasing the likelihood of issues with larger messages. Some providers such as Altice have had previous issues with size limit errors.
Key opinions
- Varying Limits: Different mail servers (sender, recipient, intermediary) have different message size limits.
- Multiple Factors: Attachments, encoding, and headers contribute to the overall email size.
- Server Configuration: Email size limits are often related to server settings and configurations, which can vary.
- Encoding impact: Encoding, like base64, increases the actual message size.
- Provider issues: Some providers have previously had issues related to message size errors.
Key considerations
- Reduce Message Size: Optimize attachments (use links instead of embedding) and reduce image sizes to decrease overall email size.
- Check Server Limits: Verify the email size limits of your sending server, recipient server, and any intermediary servers.
- Encoding: Consider the impact of encoding on the final message size.
- Test Email Size: Test email deliverability with different email sizes to identify potential issues.
Marketer view
Email marketer from Stack Overflow explains that different mail servers have different limits and that the error means the message is too large for *some* of the receiving mail servers. Reduce the size by reducing the size of attachments or using linked images.
20 Oct 2022 - Stack Overflow
Marketer view
Marketer from Email Geeks shares that there was a known issue back in March with Altice where they incorrectly rejected messages with a similar error `554 maximum message size exceeded`.
9 Feb 2023 - Email Geeks
What the experts say
2 expert opinions
The 'max message size exceeded' error can occur intermittently due to several factors. Message size limits may dynamically change based on the sender's IP/Domain reputation. Lower reputation can lead to stricter size limits. Additionally, if a message would cause a recipient's mailbox to exceed its limit, it may be rejected. Some ISPs also have historically low size limits, increasing the chance of issues with larger emails.
Key opinions
- Reputation Matters: Message size limits can vary based on IP/Domain reputation.
- Mailbox Limits: Messages exceeding recipient mailbox limits may be rejected.
- Historical Limits: Some ISPs have historically low size limits that might still affect delivery.
Key considerations
- Monitor Reputation: Maintain a good sending reputation to avoid stricter message size limits.
- Optimize Message Size: Reduce message size to minimize the risk of exceeding mailbox limits or encountering ISP restrictions.
- Check ISP Limits: Be aware of any known size limitations of specific ISPs your recipients use.
Expert view
Expert from Email Geeks explains it could be that message size varies based on IP/Domain reputation. If your reputation slides lower the limits change. It could also be that the message size would also put users over their mailbox limits and thus cause them to be full so they are rejected instead.
10 Mar 2023 - Email Geeks
Expert view
Expert from Word to the Wise explains that some ISPs had low limits historically and even if they don't reject the message outright, large messages are more likely to have issues.
13 Sep 2023 - Word to the Wise
What the documentation says
4 technical articles
The 'max message size exceeded' error arises because email systems enforce limits on message sizes. Exim uses `message_size_limit` to control incoming message size, causing SMTP failure if exceeded. RFC 5321 mandates a minimum support of 4096 octets, with larger sizes negotiated. Google Workspace has a 25 MB limit, including attachments. Microsoft sets default attachment limits in Outlook to 20 MB for Exchange Online and 25 MB for Outlook.com, but these can be adjusted by administrators.
Key findings
- Exim Limit: Exim uses `message_size_limit` to define the maximum acceptable incoming message size.
- RFC Minimum: RFC 5321 sets a minimum support of 4096 octets for SMTP servers.
- Google Workspace: Google Workspace enforces a 25 MB limit for messages, including attachments.
- Microsoft Outlook: Microsoft Outlook has default attachment size limits of 20/25 MB, configurable by administrators.
Key considerations
- Check Exim Settings: If using Exim, verify the `message_size_limit` setting to ensure it aligns with your needs.
- Size Compliance: Ensure your messages remain within the RFC 5321 minimum size requirement for broad compatibility.
- Google Limits: When sending to Gmail/Google Workspace users, adhere to the 25 MB limit.
- Outlook Limits: Be mindful of Outlook's default and potentially administrator-configured attachment size limits.
Technical article
Documentation from Microsoft states that the default maximum attachment size in Outlook is 20 MB for Exchange Online and 25 MB for Outlook.com accounts. These values can be configured by an administrator.
16 Jun 2022 - Microsoft Support
Technical article
Documentation from RFC 5321 specifies that SMTP servers SHOULD support a minimum message size of 4096 octets, including headers. Larger sizes are negotiated between servers.
12 Apr 2025 - RFC Editor
How can I diagnose and resolve email deliverability issues when emails aren't reaching intended recipients?
How can I troubleshoot email bounce issues effectively in Email Geeks?
How do I troubleshoot email bounce messages?
What are the attachment and message size limits for different mailbox providers?
What is the Google error message when an email exceeds size limits?
Why are my emails not delivering to Microsoft inboxes?
