Finally, it turned out Windows Live Hotmail was doing nothing but what I had told it to do, albeit inadvertently. Somehow, my friend's email address had made it to my list of blocked senders in Hotmail. Fortunately, unblocking her was easy.
Remove an Address from Your Blocked Senders in Windows Live Hotmail
To unblock a sender in Windows Live Hotmail:
- Select Options | More options… from the Windows Live Hotmail toolbar (or just Options if you use the classic version).
- Follow the Safe and blocked senders link under Preventing junk email.
- Click Blocked senders.
- Highlight the address you want to remove under Block Sender List:.
- Click << Remove from list.
(Updated December 2011)