Mail not Fetching New Emails

All of a sudden, Apple's Mail has stopped receiving new mails for your accounts. You can still send email, though, and you haven't changed anything in the account configuration (seriously!). What's going wrong?

This is a serious problem that has been plaguing Mail - actually, its users - for years. There are various possible reasons, of course, and I'm tired of hearing the standard piece of advice. I will repeat it for your convenience, though, but don't hit me if it doesn't work: Check your accounts, reset your passwords, re-enter your mail server configurations. Ouch.

If the standard doesn't work for you, there are two proven remedies that may apply in your case:

1. Mail generates some kind of offline cache files which may block the fetching of new mails. Delete those files and Mail should work fine again. The files are numbered "1", "2" etc., and you can find them in ~/Library/Mail/V2/IMAP-<name of your account>/.OfflineCache.

2. Fire up Terminal. Quit Mail.app. In Terminal, enter

sqlite3 ~/Library/Mail/V2/MailData/Envelope\ Index vacuum;

This will rebuild Mail's indexes and has helped on various occasions. (MacConfig)