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.