DNS Lookup failed: This system is configured to reject mail from 127.0.0.1 Selecting the "Lookup SMTP hosts in the DNS" option within the MailSite console results in you not being able to send mail to your local mail server. The following error may occur within your log files: 550 5.7.1 This system is configured to reject mail from 127.0.0.1 (DNS reverse lookup failed)
To resolve this issue you need to ensure that you enter a fully qualifying host name within your network settings:

1. Get the properties for My Computer, next click the tab; Network Identification.
2. Click the properties button
3. Under Computer name specify the HOST / Computer (example; mail)
4. Next click the More button
5. Where it says, "Primary DNS suffix of this computer; enter your primary domain name.
6. Click OK and OK again

Note: A Server reboot is usually required when making such changes