received email - address rewriting

Hi Guys
When I reply to any emails received from a particular domain, the To address is wrong. All other domains appear to OK. To particulars - email received from surname.firstname.second-initial@edumail.vic.gov.au gets changed to surname@surname.second-initial and so a reply is not deliverable. I sort of understand the rewriting for the sending user but where is the rewriting done for the receiving user?
Cheers

Nic