Here's what I got in response to my invite request:
Delivery to the following recipient failed permanently:
inbox@inbox.gmrservice.ext.google.com
Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the relay gmr-smtp-in.l.google.com by gmr-smtp-in.l.google.com. [2607:f8b0:400e:c04::e].
We recommend contacting the other email provider at postmaster@gmr-smtp-in.l.google.com for further information about the cause of this error.
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 http://support.google.com/mail/bin/answer.py?answer=6592 j1si1502294pdb.1 - gsmtp