I've had a problem for awhile with a Gmail account linked to Mail, which arose last summer after I upgraded to Lion. When I try to send a signed message (signed, in terms of security, not a signature at the end of a message), a small error message pops up for no more than half a second (way to short for me to read), but I've been able to make out the message based on a well-timed screen capture:
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 have no idea what to do in this case, as my knowledge of digital security is superficial, to say the least. I posted this question on Apple Community Discussions awhile ago, but I got no answers whatsoever. Any advice?