They receive an error message when the email is returned to them. Is this on their end?
Delivery to the following recipient failed permanently:
tonyashmore@mail.com
Technical details of permanent failure: Google tried to deliver your message, but it was rejected by the server for the recipient domain mail.com by mx01.gmx.com. [74.208.5.27].
The error that the other server returned was: 550-Requested action not taken: mailbox unavailable 550 invalid DNS MX or A/AAAA resource record
Delivery to the following recipient failed permanently:
tonyashmore@mail.com
Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain mail.com by mx01.gmx.com. [74.208.5.27].
The error that the other server returned was:
550-Requested action not taken: mailbox unavailable
550 invalid DNS MX or A/AAAA resource record
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I'm not certain who is 'they'. I assume you are referring to the person who attempted to send the email. Seems like a standard non-deliverable mail to me which usually points to an incorrect address. Should it be @gmail? In any case, it is obvious that the mail.com server rejected the email. If the address is correct, the issue needs to be taken up with them.