When I did TS for BellSouth we did block port 25 if the user was not user our domain to send the mail. In other words, if you were a BS customer and you were trying to use any other domain in a client (yahoo in OE) then we would block it. You had to use BS to send mail out. Not this did not apply to web based mail.
Here is a link about port 25 blocking that may help you out and BTW, there was no way around it unless you wet to a business acct. Setting up your own email server had mixed results with nothing 100%.
http://www.postcastserver.com/help/P..._Blocking.aspx