I've got a very basic work around but it won't prevent submission of the form, but does bring to the user's attention the wrong data has been entered.
Basically setup conditional logic hiding a message using a section break. The logic argument to show the error is:
Show [SECTION BREAK Error message] where "Email" DOES NOT CONTAIN "@usm.edu".
At the very least, it brings the problem to their attention - but doesn't properly validate the form like with "required fields" or something like that.'
Which brings up another point; an enhancement to Machform could be an additional option in the email field properties, could be to specify acceptable domain names for the emails e.g. like the file extension accept or block, you could have "accept" @gmail.com, but "block" @yahoo.com, @hotmail.com" (or whatever it might be, with a comma separated list).