IMAP diagnostics
Too many simultaneous connections: the provider is capping sessions
How it looks in the log
a2 NO Too many simultaneous connections. (Failure)The server allows a limited number of concurrent IMAP sessions per mailbox and refuses new ones. This is provider protection, not a broken transfer.
Causes, most likely first
- Parallelism set too high — trying to speed things up with threads backfires.
- Microsoft 365 caps around 20 concurrent sessions per mailbox; iCloud is noticeably lower.
- A mail client or phone is connected to the same mailbox and holding sessions of its own.
- A previous aborted run left hanging connections that take time to time out.
How to fix it
- Drop parallelism to one or two threads. On large mailboxes this is faster overall because nothing is retried.
- Disconnect mail clients using the same mailbox for the duration of the transfer.
- Wait 10–15 minutes for stale sessions to time out, then start again.
- Migrate folder by folder rather than everything at once.
FAQ
Common questions
Why not just add more threads?
The cap is on the provider side. Extra threads are refused rather than served, and the work turns into retries. One steady thread beats five that keep dropping.
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.
Code on GitHub · built on imapsync · no sign-up