Assuming you have a Word document that you are trying to send via email.....
(These directions are for Word 2003)
File - Send to - Mail recipient (or whatever)
At the end of my toolbar (should be the default toolbar because I don't
usually change them) is an Options drop-down, click on that.
Under Delivery options - Do not deliver before
Set your time and date in there. Please note: I don't have a lot of
experience with that option. I tested it last week, it did what I wanted
however it was during a time when my computer was on, connected to the
Exchange server, etc. I have no idea what factors influence the effectivity
of that option.
If the delivery time/date is that critical, resort to Tom's advice.