Shorten it up Google.

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Remote-MTA: dns;
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 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.