Yes, just the same happened to me just now. Probably just pop.gmail.com that's off-line, at least my Mail doesn't find it right now, anyway... just cancel the message, wait 5-10 minutes and try again...
The same happened to me. I deleted my account and tried to reconfigure but pop.gmail.com doesn't seem to be online. It's recomforting to know that the problem is not ot our end though
Happened to me too... just now! I ended up deleting gmail from Apple mail.
Option-drag the certificate to your desktop, double-click it and add it to x509 anchor in Keychain to prevent further pop-ups... yeah right. Didn't work.