I'm dismayed that an ISP as big as BellSouth has taken the extreme measure to block port 25 from all of their customers. If it was me, I would switch to a different ISP ASAP. I realize though that that isn't always possible.
I found an interesting explanation on their website: http://help.bellsouth.net/blscontent/co ... g_page.asp
Just FYI, FastMail will let you use port 26 too, exactly to workaround this situation. In addition, FastMail has workarounds so that you can use almost *any* port number that you want. See the SMTP section of
http://www.fastmail.fm/docs/faqparts/Ex ... verDetails
and see
http://www.fastmail.fm/docs/faqparts/Ex ... ernalProxy