I use Yahoo myself and just forward all email from my domain to my yahoo account. I set this up in cPanel.
Then in Yahoo I added my email domain as an account (you can do this in gmail and hotmail as well) so I am able to send out with email@example.com
. The last thing I did was to set my reply to and default address to my personal domain.
I repeated this step with 8 domains and now use my single Yahoo account to send and receive email from all my domains. I just pick the domain I want to send out with from my drop down list and off I go.
I have set a few people up on live mail with this and the procedure is pretty well identical, so I assume it is no different for gmail.