I ran into this issue a week or so ago...all of a sudden I was unable to connect to the mail server at this domain. Turns out SBC Yahoo was blocking port 25. I searched Google for information on how to switch Eudora 6 to use port 587. Turns out to be rather easy.
On Macintouch.com this morning there was some discussion on this issue, along with a link a support page on SBC Yahoo's site: SBC Yahoo! Help - Port 25 filters. In short, you can get this "feature" turned off by filling out a form. Not sure if I'll go this route or if I'll stick with port 587 for now.
