Message through MSG to group of recipients wrongly fails with user unknown error for all recipients
A mail targeted to multiple recipients returns with the message saying that "the following addresses had permanent fatal errors", followed by a list of all recipients. However when sending the same message to individual users it seems to work. It seems that if one recipient is invalid, all recipients get the same label, why is this happening?
When checking the message in Smart Search in the MSG Web UI, the MTA logs contains the following error: relay=, dsn=5.0.0, reply=550 5.1.1 User unknown, stat=Service unavailable
The entry from the MTA logs list a reply from a server relaying the mail to the final recipient, so the problem is not within MSG but coming from the relay server of the recipient. This is something that the recipient needs to verify on their end. If the recipient causing these errors is a legit user, they might be missing from a user verification database on the receiving end and their relay server might be set up to reject messages where any of the recipients are unknown.