I recently set up 2-factor auth on my Google account, and then went through the process of adding an application-specific password to everything.

Well, apparently everything but the SMTP entry in Mail got the news. Instead of letting me know, Mail has been trying to use my secondary Comcast SMTP, which sends anything not from Comcast into oblivion. My outgoing emails have been swiftly and silently vanishing away.

But, I tested my outgoing emails by sending to my Comcast account, natch, which worked fine because they were going *to* a Comcast address, instead of *from* a Gmail address. :-P

And not Google's fault at all. Mail wasn't connecting to the SMTP server and failed to notify me that it was happening. At root of the issue was Mail's use of the email password in two different places, with two separate copies of the password.

All I had to do was enter the password again. The same thing could happen to anyone else who changes their email password but only updates it in one place in Mail.