We migrated IMAP servers at work and for some reason I was the only one that was affected by a small problem. I couldn’t get my email! The error that would show in the maillog was:

dovecot: IMAP(user): FETCH for mailbox INBOX UID 176705 failed to read message input: Is a directory
dovecot: IMAP(user): Disconnected: BUG: Unknown internal error bytes=473/4475

After looking at several things I eventually saw that my cur directory had some subdirecties named like emails. They were empty, so I went ahead and deleted it and that fixed it!

Just thought I’d let other people know since it’s one of those things that Googling didn’t really give any answers. (Imagine that!)

 

Source: http://notjustlinux.blogspot.com/2009/05/dovecot-migration-error.html