Question about webhosting, DNS transfer and propagation

Hi

I'm having a few issues with my webhosting, and I wonder if I could pick the brain of those more knowledgeable about webhosting than I.

I've just signed up for new webhosting for my sites, and last Monday (Aussie time), my sites were migrated from my old webhosting, host A, to my new host, host B. When that was done, I then logged in to where my domains were registered, and updated my dns to point to my new hosting, and sat back to wait for it to propagate.

My websites are showing up on host B, I can ping them and the reply comes back from host B's ip address.

However, my email is still going to host A. I can't access my emails on host B, but have to login to webmail on host A to read my email.

I have created a new email address on my domain on host B. It shows up on host B webmail, but when I send a test message, it bounces back saying "no such user".

I've been told I need to wait until the dns have fully propagated, but it's been 4 days now, so it should be all done. The problem is happening with domains registered on two different registrars.

I also don't understand how a domain can partially propagate. It either points to host A or it points to host B. Is it possible that my website be on host B, but my email go to host A?

Any thoughts on how I can remedy this?

Thanks for any advice.

Audrey