Official comment
Thanks for your feature request!
What is the use case for this new trigger? We find that the similar setting in creating Groups is almost never used (Emails Received > Contains > [Name of Email]), as people prefer to go to the Messages library and click on the number "Sent" to see the contacts it was sent to. You could do that to Tag the contacts, and then have a Campaign trigger that acts on the tag added, and perhaps people are doing that currently.
