This problem has bitten me a lot while travelling:

* You suspend your laptop (while Evo is running) and come back later. Evoltuon claims to be unable to contact the IMAP server and won't until I restart it.

* If the network connection is kind of unreliable (frequent loss of connection between Access Point and WAN or whatever), Evolution freezes when you try to quit it or if you try to mark a bunch of messages as read. In other situations it takes ages to open the "Reply" window while Evolution is still kind of responsive.

I would suggest that Evolution should give up much earlier when it cannot reach the IMAP server and probably just keep track of the local changes it want to merge with the server until the next contact.