I'm on 10.8.2 and everything worked fine until today.
My POP mail account says: The server returned the error: The attempt to send data to the server “(null)” failed.
I can send but not receive mail. I could before today
My iPhone 4s says: Cannot get mail, the account is in use on another device.
Again, I can send but not receive mail but I could before today
I swithced my Mac mail from a POP account to an IMAP account and now everything seems to be working.