IMAP account via Mail

How do you specify exactly which folder(s) to use in Mail when you
have selected save on server?

I cannot find anything which will allow me to specify the exact folder
name for sent items or trash and the defaults do not store these
messages on the imap server. Do I have to edit a preferences file to
achieve this? If yes which one?

 

You first need to have folders that you can use for Drafts, Sent, Junk,
and Trash. Once you have these, from the Mail menu use Mailbox -> Use
thie Mailbox For -> and select from the list.

Dan