Sending mail from a new system. Password and userid unchanged from previous.
[2021-11-19 19:02:59] ESMTP< 535 5.7.8 Authentication credentials invalid.
** error occurred on authentication
*** Authentication failed:
535 5.7.8 Authentication credentials invalid.
Thank you for reaching out to us. I am unsure as to why you received this error. This may have been a singular event. Have you received this error since then?
It continues at this time.
FWIW, the problem happens occasionally on both my other system and my wife's. I tested it at the same time, and it did not happen.
So to summarize: a new laptop that has never been used to receive mail has a consistent authentication failure. That would be #3 system from this IP address.