No, it can't be stopped, except by not closing outlook. If you need to hold
mail like this often, use a utility that will send it later instead.
http://www.slipstick.com/mail1/recurringmail.asp#tools

Which means, although you did not mention it, that you are NOT using
Exchange as your mail server but instead are using POP or IMAP.  That
means you chose to use Outlook, an e-mail client, as a cheapie and
unreliable mail server.

If you use Exchange as your mail server, the message gets uploaded to
the mail server and it is the mail server that delivers your message
when scheduled.  You are relying on the mail server to always be up so
it can send your delayed message at the scheduled time.

If you use Outlook as a mail server, and just like any other mail
server, it must be running to do anything.  That means you must leave
Outlook running to send the message that is scheduled to be sent
sometime later.  If you exit Outlook then obviously Outlook won't be
running to send that delayed message at whatever is the scheduled
time, so you get warned that an action that you chose to perform
cannot be performed.

If you want an application to do something, it has to be running.