When you're sending through sbcglobal.net, are you using your sbcglobal.net email address, or are you using an email address from a different domain?
SBCGlobal blocks all outgoing smtp messages (on port 25, I believe) unless they're from the sbcglobal.net domain (and, sometimes, as may be in your case, they block those too). It's a way to reduce the amount of spam, and prevent people from using SBCGlobal DSL accounts from spamming people. A simple call to tech support requesting that this port 25 block be removed should fix it -- I had to do exactly that when I first got SBCglobal DSL.