RCPT TO Header rewriting

[Amelein at dantumadiel.eu]

Because of what looks to be a bug in GroupWise I need to rewrite part of the headers coming from our internal server but I am unsure of where and how to do this..
For some unknown reason the string ';1:1' gets added to the e-mail address for the DSN requests which causes our mail to bounce for a certain major isp, I started noticing this while running a tcpdump on our MS gateway.

On the incomming connection from our GroupWise system the RCPT TO line is as follows:
RCPT TO:<user at domain> NOTIFY=SUCCESS,FAILURE ORCPT=rfc822;groupwise-user at domain;1:1
When it is outbound to the internet it looks like:
RCPT TO:<user at domain> ORCPT=rfc822;groupwise-user at domain;1:1 NOTIFY=SUCCESS,FAILURE 

Somewhere along the line I need to see if I can somehow remove the ;1:1 part on our gateway as a temp fix till I hear from Novell.
Any ideas on how to do this with a MS + postfix setup ?

-
Arjan