I think gmail is as close to a "mail as a service" as you can get. You can get your own domain pretty easily, and yes Google charges you for it.
You can hook up plenty of 3rd party native clients to it - if you want, you never have to use the web interface at all (9x% of my gmail usage is from Mac and iOS native clients).
True, and I pay for it for blekko.com, but there was no amount of money I could give them that would let me have the old gmail interface back. That was sad for me.
You can hook up plenty of 3rd party native clients to it - if you want, you never have to use the web interface at all (9x% of my gmail usage is from Mac and iOS native clients).