Columba will normally send mail as soon as you click the Send button. However, there are a few ways you can save a message to be send later:
If you are offline, Columba will add your messages to the Outbox queue. The next time you are online and send or receive mail, that message will be sent when you select File->Send Unsent Messages or click the F10 function key. It will not be sent as part of a normal File->Receive and Send selection or by pressing the F9 function key.
Choose File->Save As Draft to store your messages in the drafts folder