Current versions of Outlook include an option to send immediately. Look for "Send Immediately when connected" on the Tools, Options, Mail Setup tab.
An Exchange server account configured in cached mode honors the Send immediately setting and Send & Receive settings (Ctrl+Alt+S to open the Send & Receive dialog.)
When you use a Microsoft Exchange server email account in Online mode, Outlook will always send immediately. This is a feature of the Exchange server account. You do not need to enable Send immediately, and if you want to hold mail to send later, you need to set Outlook offline.
The one thing all versions and modes have in common is that you need to have the Internet mail account or service configured to use a LAN connection. If you don't really connect to a LAN (and, remember that a DSL or cable modem connection is actually a LAN connection), you'll need to set up Internet Explorer to automatically dial the Internet and set Dial-Up Networking for an inactivity timeout so the modem does not stay connected forever.
To disable temporarily, set Outlook to Work Offline. Look for Work Offline command at the end of the Send and Receive ribbon.
Note: the icon is highlighted in orange when Outlook is in Offline mode and the status bar will also indicate you are offline.
Outlook 2007, 2003
To disable temporarily, set Outlook to Work Offline. Look for Work Offline command on the File menu.
Note: there is a check in front of the Work Offline command when Outlook is in Offline mode and the status bar will also indicate you are offline.
CW Mode or Outlook 97
Set up the Internet E-mail service on the Connection tab with the Connect using my local area network (LAN) connection. Make sure Internet Explorer is set to dial out automatically.
If you don't want to send an item immediately, close it rather than clicking Send. This will put the message in your Drafts folder.
In Tools | Options, on the Mail Delivery tab, check the box for Send messages immediately when connected.
To turn off the immediate send feature temporarily, choose File | Work Offline.