Strange and infuriating email problem.

I'm a graphic designer, working at home and have no problem in receiving emails from any of my clients --- except one.

It started about a month ago and I have no idea what's the problem.

I can send to her and she can reply to that particular email, but emails originating with her will not come through.

I use Entourage via Firefox (on an iMac). She uses Yahoo on a PC.

I've tried setting my spam filter to "none", with no results.

I've had to create a Gmail account just to get her mail. And frankly, Gmail stinks. In order to update for any messages, I have to quit Firefox, start up again and re-log into Gmail, since their "refresh" button doesn't work.

Any ideas?