Delivery to the following recipient failed permanently:
Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the relay
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.
(I did receive a confirmation of my invite request a little later, but it’s still funny 🙂 It also looks pretty cool.)