If you use
Microsoft Entourage (which is part of
Office:Mac), you have a few options to do what you want:
1. Flag your messages. You can either just click the little grey flag icon in your list of mail messages, or you can select the message, click on the "To-do" toolbar icon, and select when you want to set its due date. Your message will now have a little red flag icon next to it in your list of mail messages. Additionally, when you go into Entourage's to-do list (either on the right side of your calendar or in the task list), it will be listed there too. When I flag messages, I tend to set both a due date and a reminder.
2. Add the message to your calendar. Select the message, then go to the Script menu (it's the one to the right of the Help menu), and select "Create event from message". When you do this, the body of the email will be copied to the body of a new event. Then you can set the specifics of the event. When I opt for this method, I'm doing it to specifically schedule that time to work on something. I just did it for an email from my manager asking me to help her with a presentation, for example.
3. Add the message to your task list. This is roughly equivalent to the first option. Select the message, go to the Script menu, and select "Create task from message". Just as in the second option, the body of the email will be copied to the body of the new task, and you can set start dates, due dates, and reminders however you might like.