First up, if you're not a Gmail user, you should really get into it, it's an amazingly powerful thing to use in this technology driven world. Second, it has just been updated for iOS, now sporting a new important feature: sending e-mails from alternate e-mail addresses.
This is something quite useful for me, personally, as I'll send a bunch of e-mails every day from either my personal @gmail.com account, but I also have all of my TweakTown e-mails forwarded to my personal Gmail account so that they're synched across my smartphone, MacBook Pro, iPad, PC, and any other Internet-connected device I use, anywhere in the world. This way I can "reply as" either @gmail.com or @tweaktown.com, which is just amazingly powerful and easy to use.
The updated iOS app can now send and respond to messages from any address you have within your Gmail account, finally! But, while the new Gmail app is updated, Apple's default Mail program is still pretty powerful and something I'd recommend over Google's app, or Sparrow for iPhone which is quite powerful in itself, but doesn't support push notifications (what the?) at the moment.