You want the domain on the email in the tag you add?
In Mail Policies/Text Resources, create a Disclaimer, and you can insert some of the variables. Including From and Envelope From
Then using a content filter attach the disclaimer to the emails.
To get JUST the domain, it's possible you could use regex on the conditions of the filter (eg find the domain, make sure is isn't blank) and then use MatchedContent to put that in the Text resource.
Complicated, but might work.
________________________________
This email is intended solely for the use of the individual to whom it is addressed and may contain information that is privileged, confidential or otherwise exempt from disclosure under applicable law. If the reader of this email is not the intended recipient or the employee or agent responsible for delivering the message to the intended recipient, you are hereby notified that any dissemination, distribution, or copying of this communication is strictly prohibited.
If you have received this communication in error, please immediately notify us by telephone and return the original message to us at the listed email address.
Thank You.