GMS Mail uses first domain on system
NTMail uses first domain on system in Received clause and as the address for returned mail.
This only happens if mail is being handled by an IP address that is not registered to GMS Mail.
There are 2 ways that this could happen.
1. Either post is relaying mail, so neither sender nor recipient is local. The message cannot be delivered and post has to return it to sender.
2. Or the mail has been sent to an IP on the machine that is not registered in GMS Mail, for example 127.0.0.1
The solution to 1 is to make sure that you have completed the Administrator email address field on the System > Miscellaneous page.
The solution to 2 is to add a Domain Alias to the domain that you would like to deal with this mail of the form [127.0.0.1]. It is important to include the square brackets.