Mailplane is a desktop application that provides you with all the features of Gmail plus the advantages of a standalone e-mail client. Mailplane can access and manage multiple Gmail accounts from a single application. Mailplane supports standard OS X features.
Mailplane is a dedicated Gmail client. Unlike Mail or Entourage, Mailplane doesn’t access your Gmail account via POP or IMAP; instead, it uses the magic of WebKit, the browser code on which Safari is based, to give you the native Gmail interface in a standalone program.
“The main interface is Gmail itself, the same as if you opened it in any other browser. It’s only when you take a second look at the browser chrome do you realize that this isn’t your grandma’s Gmail. This is in fact, a full application dedicated to controlling one thing.
The toolbar provides the same type of features that any regular mail application has, such as the New, Reply, Forward, Send, and Discard buttons.”
Read more: http://mac.appstorm.net/reviews/mailplane-gmail-on-your-desktop/
mailplaneapp.com
Using Mailsmith to create a “send-only” email account: http://www.tuaw.com/2011/06/18/use-mailsmith-to-create-a-send-only-email-account/