Earthlink has been doing this for a loooong time. The reason being is that they dont want people using thier SMTP server for spamming. They prolly have it setup with mail relaying turned off, so your ...
With AOL and Earthlink now blocking all SMTP access from IP addresses deemed residential, a precedent has been set, and it's not a good one. This blocking doesn't affect every ComCast/RoadRunner/etc ...