For sending mail, it doesn’t matter if it is internally or externally–the mailhost should point to an SMTP server. If your example of internalmail.domain.com identifies an SMTP server, then that’s okay.
And that won’t address the address format issue anyway. If you want to validate the address format before sending to the SMTP server, write a wrapper service for pub.client:smtp to do so.
Set the from to a valid from e-mail address and your SMTP server will send an e-mail to that address for any errors encountered.
#webmethods-Protocol-and-Transport#Integration-Server-and-ESB#webMethods