I have been trying to sort by lastname the user list (at the right column of the main jmail page) without success but today I have seen a picture of jmail main window that shows a link for sorting participants: just the need I have (picture attached). 

I have download some versions of jmail but could not found the link below the alphabet letters, Where is it? Why I can't see it? So how can sort participants by lastname?

Thanks in advance,

Mariano