Sent emails are not stored in the specified Maildir and don't show up in search

[abid-mujtaba]

When I compose and send an email using sup (configured to use msmtp) the email is sent but it is not stored/saved in the specified Maildir and doesn't show up in search later.

I ran sup with debug log and after sending an email this line appears:

[2016-07-20 11:28:04 +0500] store the sent message (locking sent source..)

But the sent message is not stored. I have tried changing the sent_source to sup://sent but that doesn't work either.

[kalifg]

I can successfully see my sent message (label Sent); however no matter my configuration of sent_source they are sent to sup://sent.