Evolution hangs when trying to show (or to delete, it seems) a message with extremely many lines (empty lines in this case, not sure if applies for messages that have content on the lines).

To reproduce: Send yourself an E-mail with eg. 256K line feeds in it. (I tested with that amount; less will do, I first encountered this with ~232K line feeds among other data in an incoming message.) When you try to open it, Evolution will hang. It will consume a lot of CPU. It is possible that it would end up having processed the message at some point, but my patience ran out. I've deleted the messages via webmail and cleaned Evolution's message imap cache manually, and the problem has gone away for now.

Since this is an easy DOS against Evolution, and causes a hangup (for a long time if not indefinitely), I determined that it was equivalent to crashing and thus deserved "critical" severity.