Domain error on Send Mail Action

[adcprog]

When using the Send Mail action I receive the following error:
"Error sending message: The specified domain name is invalid or not recognized (5.5.4 <Pro>... Domain name required for sender address Pro )"

According to the documentation, the Domain name is optional in the Send Mail properties. We currently are not using a domain so I left the field blank. I don't know what Send Mail is looking for. Can anyone offer suggestions?