Thunderbird hangs on "copying message to sent folder"

I don't know why Thunderbird wants to save a copy of sent emails to the folder " Sent" , however, if you don't have one then the message "copying message to sent folder" hangs on forever even after the emails has been sent.

A quick fix on this, please do as follows:

(Thunderbird 31.02 ) Click on the upper right menu icon as indicated by the red rectangle
,then Options ->  Account Settings.

Choose  Copies and Folders -> Place a copy in -> Other