If you have set up your account (or accounts) to hold back outgoing mail in a queue instead of delivering it immediately, messages are not sent until you tell Opera to do so. A fast way to make Opera deliver all queued mail from all your accounts is a keyboard shortcut.
Send Queued Mail Quickly in Opera
To send queud mail swiftly in Opera:
In Opera 6, the keyboard shortcut is Ctrl-Shift-Q.