Emails sortable by headings in the Email Module?

I just noticed (because I had a reason to do so) that I can’ t sort emails by anthying except date sent/received.

Is this a bug, or is this functionality just not available?

(SuiteCRM 7)

I have no idea if it’s on purpose, but maybe you could try making the fields sortable (from Studio, or vardefs) and see if you run into any trouble…