"SMTP Error: Message exceeds server limit" when email attachment is smaller than limit

Symptoms

You have problems with sending larger email attachments and they hit the upload limit even though the attachment is smaller than the limit.

Description 

Emails are encrypted in Base64 before they are sent, which increases the size of the email. This increased size can trigger the ModSec upload limit rules and prevent the email from being sent even though it appeared to be smaller than the limit when initially attached.

Workaround

Increasing the message_size_limit in the Exim Configuration Manager is an effective solution to this problem:

1. Navigate to "Service Configuration / Exim Configuration Manager"
2. Select the "Advanced Editor"
3. Scroll to the bottom of the section labeled "CONFIG",
4. Select "Add Additional Configuration Setting"
5. From the drop-down selector, choose "message_size_limit"
6. Into the text box, type a larger limit. The default is "50M"
7. Scroll to the bottom of the "Advanced Editor"
8. Select "Save" to commit the changes

Additional Information

Base 64 MIME