There are plug-ins available for Apple Mail. Some are plug-ins, some are AppleScripts.
You could easily accomplish this via AppleScript. For example, here's a set of AppleScripts for Apple Mail that includes a delay send one, although the author of those scripts notes that they haven't been updated recently and might not work with more current versions of OS X. You'll have to try it and see. Since it's an AppleScript, if you're willing to get your hands dirty, you can edit it to better meet your needs.
Another option is Mail Scheduler, which includes delayed send as well as several other related items.