Check the HELO hostname of your mailserver. You will need to assign it a Fully Qualified Domain Name (FQDN) for the hostname setting. Simply enter any hostname which points back to an IP address on the mailserver.
Log into SmarterMail as the System Administrator
click on 'Settings' and 'General Settings'.
Change the Host name from webuser to an actual domain which points to your server
You can also check with the owner of your IP address (host or ISP) to make sure a valid Reverse DNS entry is setup.
Those two items should do the trick.
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Dustin H., MCSA
Enterhost Support Team
www.enterhost.com