I use Postbox 2.1 for my email client on the Mac. It is efficient, has advanced searching, and let's you view photo thumbnails, attachments, and todos from any folder and directly export them to another program. This makes finding that lost attachment somewhere on the server very easy.
I recently encountered a problem, however, where despite having set the account-specific preferences to not quote the original email in a reply, it was doing it anyway. No matter what I did or tweaked, Postbox would automatically included the quoted message. It was pretty annoying, as I had to delete everything manually, and for emails that include a lot of HTML that can be a pain. So if you have issues with your Postbox preferences too, you can do what I did ($ refers to command link prompt in the terminal)
You can scroll through and pretty easily figure out what the different preferences are from there name, just be sure that you have a backup in case you mess something up. Worst case scenario you just rename your prefs.js.backup back to prefs.js and you're good to go