I used my Gmail email address for almost nine months in Mail 4, and it worked swimmingly. An issue I've gotten lately with Mail 5 in Lion, is that whenever I want to send a message from my primary account, the compose window disappears as it should, but is then followed by a pop-up window from Mail, which disappears before it can be read. I isolated a frame from a ScreenFlow recording of the event, and the message reads, "An error occured while trying to sign this message with a certificate from [myusername]@gmail.com. Verify that your certificate for this address is correct, and that its private key is in your keychain." I've tried going into Mail>Preferences, and changing the security certificate. Nothing. Removing and adding the account again. Nothing. Is there anything else I can do? Receiving emails still works, and both sending and receiving works when I visit Gmail in my browser.