Why am I getting a max message size exceeded error on some emails but not others?
Michael Ko
Co-founder & CEO, Suped
Published 1 Aug 2025
Updated 16 Aug 2025
8 min read
Encountering a 'max message size exceeded' error can be frustrating, especially when some of your emails deliver perfectly fine while others bounce back with this specific error. It leads to confusion because you might wonder why the limit applies inconsistently, or if it indicates an issue with your setup or the recipient's. This common deliverability challenge highlights the complex interplay of various factors that determine whether an email, particularly one with significant content or attachments, reaches its intended inbox.
Understanding this inconsistency requires a closer look at how email systems handle message sizes, from the sending server to the final recipient's mailbox. It’s not always as simple as comparing the file size of an attachment, as the actual message size can be significantly larger due to encoding and other transmission overheads.
The good news is that by dissecting the potential causes, you can diagnose these issues and implement strategies to ensure your emails reliably reach their destination, regardless of their content size. We'll explore the technical reasons behind these errors and offer practical solutions.
Understanding email size limits and overhead
When you send an email, its 'size' isn't just the sum of its attachments. The entire email, including the header, body, and any attachments, undergoes a process called MIME (Multipurpose Internet Mail Extensions) encoding. This encoding converts binary data, like images or documents, into a text-based format that can be safely transmitted via email protocols. This conversion process typically adds about 33% to 50% to the original size of the attachments and the email body itself. Therefore, a 15MB attachment might become a 20MB or 22MB message in transit, easily pushing it over common limits.
Every email server, both sending and receiving, has configured limits for the maximum message size it will process. These limits are set by internet service providers (ISPs), mailbox providers (like Gmail,Outlook, Yahoo), and even an organization's internal mail servers. If your sending server has a higher limit than the recipient's server, an email that successfully leaves your system might still be rejected upon arrival.
The hidden size: MIME encoding
When an email is sent with attachments, the entire message, including text and binary files, is converted into a specific format using MIME encoding. This process adds a significant amount of overhead to the original file size, often increasing it by about 33% to 50%. This means a 20MB attachment could result in a total message size of 26MB to 30MB, potentially exceeding typical 25MB limits.
Binary to text: Attachments like PDFs, images, or documents are binary files. Email, however, is fundamentally text-based. MIME encoding converts these binary files into a text string that can be transmitted via standard email protocols.
Base64 encoding: A common encoding scheme is Base64, which represents binary data in an ASCII string format. This conversion always results in a larger file size than the original binary.
Headers and boundaries: Beyond the encoded content, the email also includes MIME headers and boundary strings that define different parts of the message. These add further to the overall message size.
Commonly,email providers impose message size limits ranging from 10MB to 50MB.Gmail, for example, has a 25MB limit. If your total message size, including all encoding overhead, exceeds this threshold at any point in the delivery chain, the email will be rejected. This is why you might see a 552 5.3.4 message size exceeds fixed limit error for messages with attachments, even if the attachment itself seems smaller than the stated limit.
Why limits vary: recipient, server, and reputation factors
The reason some emails get through while others don't often boils down to specific configurations or conditions on the recipient's side, or even dynamic adjustments based on your sender's reputation. Different recipient mail servers can have different maximum message size allowances. One recipient's server might accept a 30MB email, while another's might reject anything over 20MB. This disparity immediately explains why some emails successfully deliver and others bounce with a size error.
Beyond the static server limits, dynamic factors come into play. A recipient's mailbox might be nearing or at its storage capacity, causing even a moderately sized email to push it over the edge and trigger a 'size exceeded' error, even if the message itself isn't exceptionally large. Some mailbox providers, especially smaller ones, might also have more stringent or less flexible limits, or they might interpret various delivery issues as a hard bounce, including size-related rejections. This can lead to unexpected bounce classifications for a seemingly small email.
Sender-side considerations
Your ESP's limits: Your own Email Service Provider (ESP) or mail server has an outbound message size limit. If your email exceeds this, it won't even leave your system.
Email content optimization: Emails with heavy HTML, inline images coded directly into the HTML, or numerous attachments can quickly balloon in size, even before MIME encoding.
Sender reputation:
A poor or declining sender reputation might cause some receiving servers to apply stricter limits, or even throttle your sends or reject messages more readily, especially larger ones.
Recipient-side considerations
ISP/Mailbox provider limits: Different providers like Google, Yahoo, or smaller corporate mail servers have their own specific maximum incoming message size limits.
Recipient mailbox capacity: If a recipient's mailbox is almost full, even a message well within the server's typical limit could exceed their personal remaining storage and bounce back with a size error.
Temporary glitches: Occasionally, server-side glitches or temporary misconfigurations can lead to incorrect bounce messages, such as a size exceeded error for a message that isn't actually too large. This can be particularly frustrating to diagnose.
Sometimes, errors like '552 5.3.4 message size exceeds fixed limit' (as highlighted by Broadcom's knowledge base) are triggered even when the raw attachment size appears acceptable. This is often due to the MIME encoding overhead, pushing the total message size beyond the recipient's threshold. Understanding common message limits across different providers is crucial for proactive management of these issues.
Decoding bounce messages and troubleshooting steps
When an email bounces, the accompanying error message provides vital clues. A common error code for size-related rejections is 552. This signifies that the requested mail action aborted because the message storage allocation has been exceeded. While typically straightforward, as seen with some providers like Optimum, sometimes the error can be misleading or appear due to temporary system anomalies. This could explain why an email fails for some recipients but not others on the same domain during a specific time frame.
Common 'message size exceeded' error codes
552 5.3.4: This is the most frequent SMTP response for exceeding message size limits. It indicates the message was rejected by the receiving server due to its size.
550 (various subcodes): While 550 often relates to mailbox unavailability or policy rejections, some subcodes can indicate a storage quota exceeded, which effectively acts like a size limit.
Client-specific errors: Your email client (like Outlook or Thunderbird) might generate its own internal error messages before the email even attempts to send, if it detects the message is too large for your configured outgoing server limits.
To troubleshoot, first, examine the full bounce message. It often contains diagnostic information including the specific error code and sometimes even the limit imposed by the receiving server. Next, check the actual total size of your email, considering encoding overhead. If you're sending to multiple recipients at the same domain and only some bounce, it could indicate individual mailbox quota issues rather than a blanket server limit. Reviewing your sender reputation metrics can also provide insights, as some providers may apply stricter limits to senders with lower scores. This can also lead to your emails going to spam.
Optimizing email content for deliverability
The most direct way to mitigate 'max message size exceeded' errors is to optimize your email content. For emails with attachments, consider alternative delivery methods. Instead of attaching large files directly, upload them to a cloud storage service like Google Drive or Dropbox and include a link in your email. This significantly reduces the message size and bypasses most attachment limits. Ensure the link is clear and easy to find for recipients.
For image-heavy emails, compress images before embedding them. Use appropriate formats (e.g., JPEG for photos, PNG for graphics with transparency) and scale them to the size they will appear in the email, rather than embedding large, high-resolution images that are then resized by the email client. Avoid embedding images directly in the HTML using base64 encoding if possible, as this adds to the overall size. Instead, host images on a web server and link to them using standard image tags.
Best practices for email size
Compress images: Optimize all images for web use before embedding. Tools are available to reduce file size without significant loss of quality.
Externalize attachments: For large files, use cloud storage services and provide a link instead of attaching them directly. This is a robust solution to avoid size limits.
Clean HTML: Minimize unnecessary HTML code, inline styles, and excessive white space. While typically a minor factor, for very large emails, clean code helps.
Monitor your bounces: Regularly review your bounce logs for specific error codes like 552. This helps identify which recipients or domains are imposing stricter limits.
Regularly testing your email deliverability and monitoring bounce rates can help you proactively identify and address size-related issues. Staying informed about mailbox provider limits is a crucial aspect of maintaining good deliverability and avoiding blocklists (or blacklists).
Views from the trenches
Best practices
Always account for MIME encoding overhead, which can add 33-50% to your email's raw size.
Sender reputation can influence how receiving servers apply message size limits.
For large files, linking to cloud storage is almost always better than attaching them directly.
Even without attachments, image-heavy HTML emails can exceed limits, especially if images are not optimized.
Common pitfalls
Assuming the file size of attachments is the email's total size.
Not considering that recipient mailbox limits can vary or be nearly full.
Misinterpreting a 'size exceeded' error as a generic hard bounce without investigating.
Failing to monitor specific bounce codes and their causes for different ISPs.
Expert tips
Regularly audit your email templates for size optimization, focusing on image compression and efficient HTML.
Proactively check known issues or outages with major mailbox providers like Altice (Optimum).
Segment your audience based on known mailbox provider limits if large sends are frequent.
Implement a robust bounce processing system to categorize and act on size-related errors.
Expert view
Expert from Email Geeks says there was a known issue with Altice (Optimum.net's parent company) incorrectly rejecting messages with a 'maximum message size exceeded' error around March 2022.
2022-05-10 - Email Geeks
Expert view
Expert from Email Geeks says that message size limits can vary dynamically based on an IP or domain's reputation. If reputation declines, stricter limits might be enforced.
2022-05-10 - Email Geeks
Ensuring smooth email delivery
The 'max message size exceeded' error, particularly when it occurs inconsistently, can be a puzzle for email senders. It underscores that email delivery is not just about sending; it's about the entire journey your message takes, encountering various limits and conditions along the way. From your own sending server's configurations to the recipient's mailbox capacity and the policies of their email provider, numerous factors can influence whether a large email successfully lands in the inbox.
By understanding the impact of MIME encoding overhead, the varying limits of different providers, and the specific nuances of bounce messages, you gain critical insights. Proactive steps like optimizing email content, leveraging cloud storage for large files, and diligently monitoring your bounce logs are essential for maintaining high deliverability rates. Addressing these issues not only ensures your messages arrive but also safeguards your sender reputation against negative impacts.
Outlook, Yahoo), and even an organization's internal mail servers. If your sending server has a higher limit than the recipient's server, an email that successfully leaves your system might still be rejected upon arrival.
Gmail, for example, has a 25MB limit. If your total message size, including all encoding overhead, exceeds this threshold at any point in the delivery chain, the email will be rejected. This is why you might see a 552 5.3.4 message size exceeds fixed limit error for messages with attachments, even if the attachment itself seems smaller than the stated limit.
Google, Yahoo, or smaller corporate mail servers have their own specific maximum incoming message size limits.
Optimum, sometimes the error can be misleading or appear due to temporary system anomalies. This could explain why an email fails for some recipients but not others on the same domain during a specific time frame.
Outlook or Thunderbird) might generate its own internal error messages before the email even attempts to send, if it detects the message is too large for your configured outgoing server limits.
