Well, I don't know how many people you have that you want to give email, but there is an unlimited number of emails that you can use.
You cannot "mix" servers, in the strictest sense of the word. The site is on the server that the DNS (Domain Name Server) says it's on. If you change the DNS, you change the server that the domain name "points to", so to speak.
I hope that clears things up. The short answer is: Switch servers and run everything off of one server.
EDIT: It is possible to run your email on another server, but probably not worth the hassle. If your time is worth something, just get a better server.
-Michael