Well, Ive seen how it works, with imap threads extension. I dont know the syntax of queries and maybe the server implementation could take care about this (Dovecot in my case). Any way the roundcube imap client could also do recursive queries of headers just to show the full conversations tree and maybe store the relationships in the db (caching).
But for me and now, the fastest workaround I found, was to use INBOX for sent_mbox (and maybe drafts_mbox). It was disabled by default (dont know why), I had to change a file :
program/steps/settings/func.inc -> function rcmail_user_prefs -> case 'folders': -> $select = rcmail_mailbox_select(array('noselection' => '---', 'realnames' => true, 'maxlength' => 30, 'exceptions' => array(/*'INBOX'*/)));
This could have further implications : maybe filters will also now apply to sent mails,... let's see. But conversations are very important for me and yes, just another suggestion : the message view could show the tipical mailing lists buttons : previous/next by date/thread.