I previously had a problem with messages remaining in the outbox and NOT sending. This seems to have resolved itself on its own.
Now I have a similar problem but instead of the emails not being sent they are being sent numerous times. They show in my folder as sent only once but a recipient has contacted me to say they have received the same email 32 times!! I checked and the email shows as being sent once only my end but was bold in the outbox ( as if unopened and unsent) so I have deleted it to avoid it being sent for the 33rd time!
This is most likely caused by your server, which is not giving the correct response when you send the message. Therefore eM Client sends it again, and again, util the proper response is received. If you go to Menu > Tools > Operations, you will find the SMTP error in the Log tab.
Who is your email provider?
Right now you can stop the message being sent by deleting it from the Outbox.