create a new reseller. Under that reseller create two users, each with a different subdomain of your domain: for example if you've got a domain on the server example.com, then create test1.example.com, and test2.example.com, both under the new reseller.
Then suspend test1.
Then send a message to all users, and check to see if only test2 gets it, or if both get it.
You're right of course. If the email address is at the suspended domain.
But that wasn't the question. The question was, does DirectAdmin send the email. That's important because the mailing address for the suspended domain may be in fact hosted elsewhere.