[imp] right message-list header for additional folders

[li at elaboratio.com]

Hello,

I have an additional folder in my /home/usr/mail directory called
"sent-mail_old" which I transferred from an old server.

Imp shows the messages but uses the header template from INBOX, i.e. it shows
"To" instead of "From".

Could somebody indicate me the right file which I have to modify to add some
lines to tell imp to use the template for "sent-mail" for this extra folder?

Thank you.