You can have GMail handle your email even if Google is not hosting your web site. See this document for detailed instructions to fit MX Records on GoDaddy.com DNS here: http://support.google.com/a/bin/answer.py?hl=en&answer=33353&topic=1611273&ctx=topic. There are like instruction sheets for many other registrars here http://support.google.com/a/bin/topic.py?hl=en&topic=1611273. There was also a *VERY* handy tool that looked up the current, working, DNS MX Records for me. It is located here http://www.dnsstuff.com/tools. There are several tools on this page: you want the one called “DNS Lookup”. Enter the domain and click submit. It wants you to join (pay) HOWEVER if you run the tool from the Google help page at http://support.google.com/a/bin/answer.py?hl=en&answer=140038 it is free.
Finally! Started to change the DNS for a customer at about 18:00 and after the switch was in progress he mentioned his email is hosted on GMail, not his old web host. Now I should have ASKED that question before we began, but we backed out of the switch and I started digging. And digging. And Finally, I Found Instructions at 24:00 for setting up the MX record to point to Google GMail. And very NICE instructions they are, finely documented with pictures and nicely typeset.
The one I needed was for GoDaddy, but there is an extensive list here http://support.google.com/a/bin/topic.py?hl=en&topic=1611273. If you are not clear on what an MX record is, or why it is needed, this explanation http://support.google.com/a/bin/answer.py?hl=en&answer=140038 is very nice.
So if you are really constrained by your email host, you might consider using GMail: it will still show up as your domain name, but it will work oh so much nicer. And since I have done the research for you, you won’t need to spend 6 hours digging.
Hope this Helps!