Evil message (seen in Thunderbird):

Sending of password for user *@gmail.com did not succeed. Mail server pop.gmail.com responded: Web login required: https://support.google.com/mail/answer/78754

Attempt to fix/solve it:

Try https://accounts.google.com/b/0/DisplayUnlockCaptcha

if that does not work, maybe turn Off less secure apps https://myaccount.google.com/lesssecureapps and try again Unlock page above.