Domain error on Send Mail Action
 

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 ... 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?

The Domain field is optional as far as VBP is concerned (you don't have to enter a value in that field), but your email server is saying that a domain is required for the sender email address being used. The person who administers your SMTP server should be able to help determine what the domain should be (or if you should specify a different sender email address).