Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain google.com by aspmx.l.google.com. [2607:f8b0:400d:c04::1a].
The error that the other server returned was:
550-5.2.1 The user you are trying to contact is receiving mail at a rate that
550-5.2.1 prevents additional messages from being delivered. For more
550-5.2.1 information, please visit
550 5.2.1'The Gmail user you are trying to contact is receiving mail at a rate...' - Gmail Help96si8867063qgb.91 - gsmtp