Problem
You occasionally have messages that won't process, and are using an IMAP mail server.
Folder Closed / Repeat Processing
IMAP is touted as a more advanced protocol than POP, which is true in some regards, but in some scenarios it can be problematic (see link below). If you experience FolderClosedExceptions,
Message Removed
If you have concurrent access to a common mailbox its possible that a message may be 'removed' by another handler process, the error is likely to be harmless, unless the configuration caused the message to be dropped.
Performance Issues
We have found that there can be significant performance problems with IMAP when it is dealing with large messages. Take a look at our POP vs IMAP performance page, where we also give advice on what you can do to resolve the problem:
Solution
Check your mailbox for large messages. It is possible they are not able to be downloaded in time, possibly being timed-out by a mail server or JIRA.
Consider trying POP. It is better suited for JIRA usage as mail handlers require reliable transmission of the entire email, rather than partial transmission which is what IMAP is useful for.