Mail.app refuses to set accounts online This started happening recently (not sure if it was since the last Mavericks update but I think so). I have two IMAP accounts and one SMTP account.

Mail.app seems to get stuck in offline mode if there is any network hiccup, and it refuses to go back online. The only way I can get mail to be sent or received is to quit Mail.app and open it again, and it works great until the next network hiccup.

Other network operations work fine (Safari, Chrome, ssh, etc.) -- it's just Mail.app that exhibits this behaviour.

Has anyone seen this before, or does anyone have any comments on additional things to check to help troubleshoot the issue?
What causes mail to go offline is authentication errors.  You must contact whomever is in charge of your mail authentication servers and ask them to resolve the issue. At my work mail going offline is almost considered normal due to the fact that our internet access must pass through our network proxies which often cause mail connections to fail authentication.