Home Forums Networks & Cross-Platform Networking weird email problem

I can email my brother several states away, but when I try to email this one person locally, the library where she works, I keep getting an error message from ATT about "cannot use this outbound.att.net" SMPT server. I don't see how geography should have anything to do with this. Mail's Connection Doctor tells me everything is working fine.

It happened last week too, but after trying throughout the day, the rejected local email finally went through.