With the default settings, once you click Send the message should be uploaded immediately to your email provider. There is not much chance of stopping it.
But if the message is stuck, it will be in the Local Folder Outbox. Go to Menu > Tools > Settings > General and tick Show Local Folders. Then go to that Outbox and delete the message,
thank you very much for such a fast answer. the message is not in My outbox but it keeps trying to go and fails, in fact NOTHING is in my outbox. What do I do now?
Once the message has been successfully passed to your server, it will be moved from the Outbox to the Sent folder, then eM Client has completed it’s task. Next it is up to the server to deliver the message.
Usually the server will say that it will try for a few days and then give up. There is nothing you can do to terminate that.