email - Alternative Slick/Smart method to disply Recursive Replying Comments?

I want to avoid the classical way of replying into replying into replying that goes like this

    Comment

    --> Reply 1
    --> Reply 2
    ---->Reply 3
    ---->Reply 4
    ------->Reply 5
    ---->Reply 6

I will be expecting lots and lots of replying levels and the horizontal space is limited.

So I am looking for any good alternatives to that matter. Any ideas?