This is exactly why I continue to use Gmail in the browser:
No other mail application on the desktop helps me to this degree. And Gmail's web app has an undo send feature. Undo send! (It's in Labs; turn it on!) This has saved my ass countless times. No other email app saves my ass like Gmail's web app.
I sincerely wish Apple (or the next brave soul to attempt an email client — Sparrow guys, I'm looking at you) would just write down all the cool stuff Gmail's web app does — 2-step verification? Hells yeah! — and build it right in to their app.
Until then, I stick with Gmail's web app. It saves my ass.