Pine is no longer developed by the University of Washington and is not a fully supported PCN product. It may not be 100% usable in all circumstances. Saving sent mail is one of those instances where your mileage may vary when it comes to usage.
Due to some bugs in Pine we suggest that you do not set up saved messages, however of you really want your sent mail saved, (and you don't mind the potential of pine freezing) do the following:
Edit the file .pinerc in your home directory, and replace the line
default-fcc= with default-fcc=sent-mail
Or, open pine, select the “Setup” menu, select “Config”, and change the line for default-fcc to “sent-mail”.