If Mail.app is stuck at “Fetching Mail….” with Gmail POP accounts after upgrading to OS X 10.10.3’s beta, be aware that Apple added OAuth support to Mail.app. Which is nice, except in the current beta, they don’t warn you to go set up OAuth permissions for every account. Mail.app will choke on fetching mail, without letting you know the actual problem. This is possibly POP3-specific, as I didn’t notice IMAP having this issue.
In the connection log:
[kCFStreamSocketSecurityLevelTLSv1_0] -- host:pop.gmail.com -- port:995 -- socket:0xxxxxx -- thread:0xxxxx
-ERR invalid SASL argument ......
The solution here is to simply set up Google OAuth via System Preferences -> Internet Accounts. It should automatically prompt you for OAuth permission to your Google Account when you click on the affected account in the prefpane. Once that is done, Mail.app will fetch Gmail via POP3 again.
I’ve also unticked “automatically detect and maintain account settings” under Advanced, in Mail.app’s accounts panel, though this might just be superstition rather than actual prevention of issues. I’m just old school and uncomfortable with the thought of “automagic” happening in my mail server settings.
If you’ve already given up and swapped to IMAP, but would like to come back to the Dark Side of email retrieval technology, see also:
How to setup a Gmail POP3 account from scratch on 10.10.3 beta
#127: Mail.app stuck at fetching mail on new OS X
I really need to rebuild Selfsolved. Someday, when I’m not trying to write.