in RT 2.0.14 we loved that RT keeps threading information well. It gave
"In-Reply-To" an “References” Header correctly to customer and admin
emails, so MUAs which support threading, displayed the email correspondance
Since RT3 the feature has gone. There is always exactly one "In-Reply-To"
Header to a artificial message id rt@, hence threading
in MUAs does not work.
Will you give us the threading feature again?