I can't email my sister using OE

I’ve been using gmail with OE for years and I never had a problem. My sister has an email account with embarqmail and every time I email her, I get back the message:

When I use gmail by logging onto the site and not using OE, it works just fine.

Any idea what’s wrong?

Just to be clear…you are sending using a GMail account via Outlook express, right? Or do you use some other domain with OE?

Yes, using a GMail account via Outlook express.