Remote-MTA: dns; gmail-smtp-in.l.google.com
Diagnostic-Code: smtp; 550-5.1.1 The email account that you tried to reach does
not exist. Please try 550-5.1.1 double-checking the recipient's email
address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1
https://support.google.com/mail/?p=NoSuchUser 29si104140jap.17 - gsmtp
Just make it say, we don't understand the SMTP RFC's.
https://old.reddit.com/r/sysadmin/comments/kdtag7/gmail_issues_511_bounces_again/
As always, details for when that link does not outlast my blog.
It would seem Google's Gmail failed and caused valid recipients to give a hard 5.1.1 error on 12/15/2020.