Problem with Return-Path from certain ISPs

[Ricardo Branco]

Centos+MailScanner+Sendmail+DovecotLDA

We started noticing issues with return-path not being passed though to 
Sendmail/DovecotLDA.
This causes a knock on affect on Sieve filters in DovecotLDA which rely 
on Return-Path being passed though.

If I take MailScanner out the loop then all is well, Return-Path exists 
to my mailbox, but if MS is running then it does not come through.

Any reason why this would be happening?