Tastes a little like stupid.
I am a Mac user and I use their spanky .Mac for my e-mailing pleasure. One of my friends can't ever send me e-mail from her yahoo account - regardless of the ISP and/or IP Address. Other Yahoo-ers have no problem. I went to .Mac support to see if I could find a way to make sure my settings weren't to blame. This is what I found:
Steps the recipient (.Mac member) can take
If all else fails, the recipient .Mac member can contact .Mac Support. Fill out the support form here. Make sure you include the full header and body of the message that was not received (in Mail in Mac OS X, for example, you can open the message and press Shift-Command-H to see the long headers; in Webmail, click the "Show Long Headers" link in the top right). Paste the message headers and body in the Support form.
(emphasis mine)
::headscratch::
Steps the recipient (.Mac member) can take
If all else fails, the recipient .Mac member can contact .Mac Support. Fill out the support form here. Make sure you include the full header and body of the message that was not received (in Mail in Mac OS X, for example, you can open the message and press Shift-Command-H to see the long headers; in Webmail, click the "Show Long Headers" link in the top right). Paste the message headers and body in the Support form.
(emphasis mine)
::headscratch::