I started this thread last week but prematurely declared the problem as fixed when in fact it wasn't, so in case that thread has been archived, I thought it best to start another.
The problem is the same as before. Incoming mail is being picked up fine in Gmail, but outgoing mail appears to be sent only to be followed, minutes later, by an email with the error message shown below.
For the avoidance of doubt, the two other non-Gmail accounts I have setup in Gmail can send mail without issue. Logging into Talktalk webmail, confirming I'm using the correct login details, I can send mail by that method.
I rang a friend who is a Talktalk customer and whom I know uses Gmail to handle his mail. Guess what? He was about to ring me because he has been getting the same authentication errors. He claimed the problem as been coming and going but I can't confirm how true that is.
On entering the Talktalk account details, the Gmail setup wizard chooses the following security settings automatically, which IIRC, are as per the Talktalk website.
The fact that my other accounts work fine, combined with the fact that my friend is having the same problem, make me think this must be a Talktalk issue and not a user fault.
Come on Talktalk. Please fix this ...