Any time a certain friend of mine sends me an e-mail, it does not come through to my inbox or to my spam foler. She then gets a message saying "Persistent Transient Failure: Delivery time expired". She's not on my blocklist and I've never marked one of her messages as spam - even if I had, her messages would at least show up in my spam folder, and that isn't happening. Her e-mail domain is bellsouth.net.

What can I do to fix this? Is the problem in my e-mail account or in hers?
I'm refering to my Yahoo! Mail account. It's yahoo's new domain, ymail.com.

Is it your Yahoo email, or a work server? If it's work then her domain might not be recognized as legitimate by your firewall. If it's Yahoo, then I have no idea... Sorry!