Don't schedule mail polls. When the mail is successfully received by
the mail host, Outlook is going to delete the copy from its Outbox
folder (and move it to another folder, usually Sent Items). If you
disable the scheduling of mail polls, you'll have to manually initiate
the mail poll when you want to send items sitting in the Outbox folder
as well as check for new items to receive from your mailbox up on the
server.
If you don't want your messages sent immediately, then disable "Send
immediately when connected" on the page you'll see if you click
Tools>Options>Mail Setup. If you mean you want your messages to remain in
the Outbox when you send, that's just not going to happen. The Outbox only
holds messages _waiting_ to be sent. It never contains messages that have
been sent already (unless something's wrong).