Current versions of Outlook include an option to send immediately. Look for "Send Immediately when connected" on the File, Options, Advanced page.
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.
Outlook 2010 and newer
To disable temporarily, set Outlook to Work Offline; it's on the far right of the Send/Receive tab. Note: the icon design has a red x on it. The icon is highlighted when Outlook is in Offline mode and the status bar will also indicate you are offline.
If you want a short delay or to defer sending messages until later, see
To delay sending a message in Outlook.
Outlook 2007, 2003
In Microsoft Outlook 2007 and Outlook 2003, the Send Immediately when connected option is in Tools, Options, Mail Setup tab.
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.