txt.att.net 405: Service Temporarily Unavailable
The txt.att.net and mms.att.net gateways have been failing with error 405: service temporarily unavailable. I provide a service that sends notifications via e-mail to SMS gateways, and as these notifications are time-sensitive, this poses a problem for my users who use AT&T as their carrier. My mail server is not on common blacklists and does not spam; the notifications are opt-in by the user. Please advise how this can be fixed.