IMAP diagnostics
message too large: the destination rejected one message
How it looks in the log
a7 NO [LIMIT] The message being appended is too large.The destination server refused one specific message because of its size. The rest of the transfer continues normally — only that message is skipped.
Causes, most likely first
- Microsoft 365 accepts messages up to 35 MB by default, and this is a tenant setting.
- Gmail caps inbound at around 25 MB; some hosting providers stop at 10 MB.
- A message with several large attachments grows by roughly a third again due to base64 encoding.
How to fix it
- Nothing is broken: skipped messages are listed individually in the final report.
- Raise the limit at the destination if you have admin access, then run a second pass.
- Move oversized messages manually or save their attachments separately.
FAQ
Common questions
How many messages are usually affected?
A handful in a typical working mailbox — presentations, video, archives. They are named in the report, so you can decide what to do with them after the migration.
Other errors
Find out how big your mailbox is. It costs nothing.
Connect two servers, look at the size and decide. Or take the client and keep everything on your side.
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