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.