All mail messages sent via SMTP should comply with the format username@domainname. Many mail clients, like Pegasus or Eudora, allow you to send mail in improper format (to "Steve",
for example). You can configure these clients to add the @ sign and the domain name to the end of the user name so that it will be sent to its mail server (Post.Office) in the format username@domainname.
If you do not configure your client in such a way, then Post.Office will receive the E-mail with just the user name (Steve). This cannot be delivered since it is not a valid SMTP
address. Post.Office will attempt to remedy the problem by appending the domain name found in the Address Completion domain field of the System Configuration Form.
In the event that there is no domain listed in this field, Post.Office makes its best guess by looking up the machine name and appending the domain name to it, e.g.
firstname.lastname@example.org (see manual 6-10).
Some sites have not defined these fully qualified addresses for their users and so mail is returned to Postmaster as an unknown account.
So, assuming that your E-mail clients are not adding yourpostofficehostname.yourdomainname to the end of the To: and From: lines (remember that some clients can be configured to do this completion also), it should be
as simple as adding your domain name to the Address Completion domain field in the System Configuration Form to avoid these errors.
If you have specified an Address Completion Domain in Post.Office, then check to see if your client is attempting completion.