FWIW, AT&T does not suggest using the txt.att.net email for anything critical. In a previous role I handled public safety CAD, and AT&T (and Verizon, if I remember correctly) made it very clear that if you want to be able to send anything other than a miniscule volume of texts through the email-to-SMS gateway, you would need to pay for their enterprise messaging service.

If the public safety agency/agencies in question subscribe to the FirstNet service, they should be able to reach out to their principal consultant (account team) and get a good resolution.

V/r