With 'Send Later' (paid feature) you can set a specific time when you want your message to be sent.

You can either use the drop-down menu under the Send button or via Menu > Message > Send later.

 Until the message is sent it is kept in your local Outbox folder.

The Outbox folder can be accessed through Local or Favorite Folders. If neither of these is available, you can enable them in Menu > Settings > General > Folders.

If you want to change or stop the 'delayed' message from being sent, you can edit or remove it from the Outbox.

Note: You can also set a 'default' Send later time for all your messages in Menu > Settings > Mail > Send. 

In eM Client 9 and older the Local and Favorite folders can be enabled in Menu > Settings > General > General.