Hey John,
By default, the hostname is used on the SMTP banner. This needs to match what is on your RDNS otherwise you can end up with delivery issues and your IP can be blacklisted.
I recommend setting this to whatever matches the RDNS on your server.
Please let me know if this helps.
Kind Regards,
Zach Sylvester
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com