How can I send a mail message later? On Mountain Lion
If you're using Gmail and are willing to use the (I think very awesome) web interface, check out Boomerang for Gmail. It does everything you're asking for.

EDIT: 

You can also use Automator to schedule (by delaying) when your message is sent. 

The Pause function is very helpful, and Apple has a whole section for Mail in the library. I would save it as an application and then run it with Spotlight or Alfred or Launchbar whenever you need it.