txt.att.net operators

We have been struggling to locate someone at AT&T that handles the txt.att.net servers. We have clients in our data center that can no longer send emails to mobile phones via 10digit@txt.att.net. We have contacted AT&T and they say there is no problem on their end. We can ping the server, but simply cannot connect to port 25. We have checked all firewalls of each client. Some ranges of IPs work and others don't. Looking for someone with a clue who can assist. Mike Walter