Mail has been giving me fits. First I can't add IMAP accounts on non ssl servers. I get a generic error message. Next Mail stopped letting me reply on my Exchange account. I read on here to clear my plist and that did not help, so I dug deeper and found out that it was a signature issue related to images in my signature. I deleted my signature and added it back. That fixed it for two days, now it has happened again. I guess I won't use a signature for a while. I hope they get this fixed soon!