In addition to updating Mac OS X itself, Apple has released an update to Mac OS X Server, to version 10.1.4. The Release Notes have this to says:
Mac OS X Server 10.1.4 update delivers improvements to Mail services.
Enhancements include
- Improved SMTP relay exchange for sending mail to certain mail servers.
- Allows delivery of mail from remote hosts which do not specify a name.
You can download the OS X Server Update from Appleis Knowledge Base site.