When you create a message and set the option to not send it until a future date, you'll always get a message when you close Outlook
Are you sure you want to exit without sending messages?
While it may be clear to you that there is a message waiting to be sent days later, the close process has no way of knowing how far out the message is scheduled for, so it pops up the warning every time you close Outlook.
Solution: Don't close Outlook. :) We recommend leaving Outlook open when the computer is turned on and hibernating the computer instead of shutting it down.
I certainly would not use this feature to schedule messages far in advance, not only because of the annoying alert every time you close Outlook, but because the message will be dated the day you sent it to the Outbox, not the day Outlook actually hands it off to the SMTP server. Set up Windows Task Scheduler to pop up a prepared template a week to a few days in advance and send it to the Outbox then. See Send Reports on Schedule for one method to schedule messages.
Note that Outlook must be open to for the message to be sent, which is partly why it reminds you of unsent messages every time you close it. The other reason, while less prevalent in this age of "always on" broadband, was to remind users who did not use Send Immediately or Scheduled Sends that there were messages that hadn't been sent yet. It was a lifesaver for many in the days of dial-up.
Also note that Outlook 2010 RTM lacked this warning, which annoyed users who counted on it for the reminder. This bug was fixed in Outlook 2010 SP1.
Published September 15, 2011. Last updated on April 22, 2016.