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.
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.