We see that you've been removed from DiscApps 39627, 55668, 206425, and 239395.
Regarding your situation. It appears that AT&T has classified disc.yourwebapps.com as a spammer and automatically tells us to remove their customers' email addresses.
There are 2 solutions. One, you could try contacting AT&T and tell them to whitelist disc.yourwebapps.com (IP addresss 188.8.131.52) or you could switch to a different email provider. We abide by all industry standards for sending non-spam email. All of the subscribers are confirmed subscriptions and all of our messages are SPF and DKIM compliant. Personally, I find it easier to switch email providers. I like Google's Gmail the best.