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
Software Developer
SmarterTools Inc.
www.smartertools.com