It's not the worst piece of software but I have now been trained to use CMD-Enter* as a shortcut for sending a composed message.
This only becomes a problem when I go to send a personal email using Mail.app. Mac OS X keyboard shortcuts to the rescue!
- System preferences -> Keyboard & mouse.
- Keyboard shortcuts.
- Add application -> Mail.
- Enter "Send" (without the quotes) as the menu item.
- Enter your preferred shortcut. (CMD-Enter* in this case).
On the next restart of Mail.app it will use the new shortcut.
*Where CMD is the ⌘ key.