For a while I was using Mail.app to POP my gmail account. Sending messages that way worked fine. Since then I have stopped using POP, and deleted that account from Mail. When I send something from my @mac address I get the error about smtp.mac.com. However it still remembers the gmail smtp information, and I can tell it to send through that.
That works, but it just bothers me that I can't use smtp.mac.com 😕