Hmm, I use Eudora not Apple’s Mail.app, but it seems there trouble abroad for Mail users trying to send emails to Hotmail users:
Seems Microsoft’s Hotmail servers are not properly passing emails sent from people using Mail.app. … A user will send an email via one email client (Thunderbird, for instance) and it will go through fine, but an identical email sent via Mail.app just disappears into the ether. … Microsoft appears to be aware of the problem (according to some ISP’s) and is working on a fix.
[Via The Unofficial Apple Weblog.]