To prevent spam each private server is setup with poprelay, a program used to authenticate users. The following conditions must be met in order to send mail through a domain:
1) You must check mail within 15 minutes of sending
99% of all connections will authenticate automatically after checking your pop. there may be isolated instances where poprelay cannot authenticate your ip. If this occurs you could make a entry in the sendmail hostfile.
To make an entry goto the services link in the admin web GUI and click on the pencil (edit icon) next to sendmail. Click on 'configure' and add your IP to the host file and hit submit.
If you don't know what to enter for an IP, contact our support dept for assistance.