Comcast: Excessive failed authentication, ESMTP server temporarily not available.
This is a solution for a problem that has recently arisen. After upgrading to TB 78.6.0, I began to get errors like in the subject line above whereby inbound service worked fine but outbound crashed. After much fiddling about with Win10 security settings (virus, firewall, etc), I noted that wife's separate acct had similar issues (thus, not machine specific) and that outbound service still worked from Comcast's online app (thus, not truly an acct issue).
I finally stumbled on the solution: in TB Outgoing Server settings, I had, for some arcane reason, two smtp.Comcast.net servers listed. The Default one had my actual username listed for username, while the other had my "extended" full username@comcast.net. Apparently, TB was trying to authenticate with the second, full address. When I edited the username and dropped the "@comcast.net," TB works.
Since the same fix worked on my wife's separate acct with the same issue, I presume the latest TB update (78.6.0) must have tightened some security authentication or some similar scenario was implemented at Comcast.
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The Comcast site indicates the User Name is your 'Comcast username', i.e. the part of the address before @, and this is shown in the pictures for Outlook 2013 or 2016. But the instructions for TB 16+ suggest the User Name is the full address, which is the same in the TB ISPDB, the registry applied when setting up a new account.
I suspect Comcast made the change long after TB 16 was current, and the ISPDB has not been updated to reflect this.
Hmm. Looks like my yesterday forum reply didn't post. It said that after the username was modified, everything worked. But yesterday morning, the same failure returned for both my wife's and my acct, thus suggesting it was neither machine nor TB specific but instead, was a Comcast issue. After a long session with a congenial Tier 1 tech, it got a ticket and was passed to Tier 2 with instructions to wait 24-48 hrs and they'd call me back. Today, with no communications from Comcast, I tested TB outbound. A box popped up asking for my Comcast pwd (suggesting Comcast reset a login jam?) and TB outbound is again functioning.
Still no joy. In further diagnoses, I've noted a trend of send failure during daytime hours but am able to successfully resend the same mssgs later in the evening. I'm now thinking that since this error occurs intermittently on three household machines and two accts, it is unlikely to be a machine- or TB-client-related issue. A Sys Admin friend informed me that on the system she maintains, the server sends this error when there's an overload of requests. However, after spending hours with friendly Comcast support techs and two support tickets, the final Tier 2 tech shut me down saying that since Comcast webmail works and they don't support TB, it's not their problem. No where to go next. Frustrating.