Why doesn't Apple Mail check my mailbox?
If the Apple Mail program doesn’t check your mailbox but doesn’t show any error messages, the account has probably been marked “offline” in the Mail program.
You can usually identify an offline mailbox by a “wavy line” icon next to the mailbox name, like this:
To fix this, choose Take All Accounts Online from the “Mailbox” menu.
(Some versions of Mail may have this item in a different menu; look through all the menus if you can’t find it under “Mailbox”.)