Why am I getting a max message size exceeded error on some emails but not others?
Summary
What email marketers say10Marketer opinions
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.
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`.
Email marketer from Reddit explains that the size limits could be imposed by the sender's mail server, the recipient's mail server, or an intermediary server. The actual maximums may vary.
Email marketer from EmailDeliveryJedi explains that 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.
Email marketer from Gmass shares that there is a fundamental discrepancy between what email providers *say* their limits are and what they *actually* are.
Email marketer from Mailjet explains that Factors such as encoding, headers, and attachments all affect the overall size of an email, potentially leading to the error even if the attachment itself seems small.
Email marketer from cPanel Forum shares that the error occurs when trying to send an email that is larger than your mail server allows. They advise checking the mail server settings for the maximum allowed size.
Email marketer from Microsoft Support explains that the "552 5.3.4 Message size exceeds fixed maximum message size" error means that the message is too large to be accepted, either because the message as a whole is too large, or an individual attachment is too large.
Email marketer from ServerFault explains that the client's ISP may have different size limits configured.
Email marketer from SuperUser shares that the encoding of the email will affect the eventual size. For example base64 encoding increases it by about 33%.
What the experts say2Expert opinions
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.
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.
What the documentation says4Technical articles
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.
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.
Documentation from Google Workspace Admin Help explains that the maximum size for messages sent or received through Gmail is 25 MB. This includes attachments.
Documentation from Exim explains that the `message_size_limit` setting controls the maximum size of an incoming message that Exim will accept. If a message exceeds this, the SMTP command will fail.