[X4U] use this mailbox for..

[Kansas Territory]

I remember at some point setting up my mail.app  telling it to use  
"this" folder for sent mail,  and this other folder for delete  
messages etc..

I'm trying to make some changes to my mail.app  and I'm wanting to  
designate a new folder to be used for SENT MAIL, but when I select  
this folder and go to the Mailbox menu to where it says "use this  
mailbox for.."  all the options are grayed out and won't let me  
select anything.

Any suggestions ?

Kansast