One cow is unique. Chances are, two are unique as well.
"Kine" may, after all, be the only plural word in English (saving pronouns) that shares not one letter with its singular form (though stemming from the same root as "cow").
Now, most email programs make it seem like sending a reply and dealing with the original message have nothing in common. In Sparrow, though, you can press one single letter (plus two function keys) to archive and deliver:
›› After sending a reply, you archive the original message more often than not? Here's how to get a button (and keyboard shortcut) in Sparrow that delivers your reply (or forward) to its recipient and the original message to your archiving folder.