I think maybe Google has tightened up security or something. I got the "could not authenticate" message though the username and password were correct both times.
I got a message from Google security about the access - but there was no way to whitelist it specifically.
Then I found this:
https://support.google.com/accounts/answer/6010255
I had to turn on the access for less secure apps.
Now emails are working! However, now my Google account is less secure. :(
Wish there was a way to whitelist just the plugin access... Any ideas?