Delete mail in local folder of mail to be send

I copy sent messages in local folder for conservation, but erroneously I paste them in local folder of messeges to be sent intead in send messages.
Now in local folder of mail to send are 1262 (number colored in red) messages seams are in waiting to be sent.
I whant do delete them. How can I do ?

Select the messages and delete.

I suggest that you turn off conversations (Menu > View > Conversations) before you do that.