Thank you for the inquiry. I would take a look at the General Settings for the Server Info to see what you have in the Hostname field. I presume the issue is the Hostname is not using a fully qualified domain name and the error is presented.
Log into SmarterMail as the System Administrator. Click on Settings | General Settings | Server Info | Hostname.
Please ensure you have the full address, such as [mail.domain.com] in the hostname field. This should resolve the issue with you sending to various web addresses.
Please let us know if you need further assistance.