Hello all,
I need to implement one feature in postfix. We have abc.com domain. Now we have assigned a new domain xyz.com. Both the domains are currenty working. Mail server is implemented in such a way that if any mail comes to user@xyz.com it will be forwarded to user@abc.com. Now I want that if any mail come to user@xyz.com is replied from the id user@abc.com then in the from address it should automatically be written as it replied from user@xyz.com. I hope you understand what I am asking. I am not sure if it is a postfix issue or not. Please help me for solving this problem. waiting for your reply...

Just to clarify what you want:
Mail comes in to user@xyz.com (rcpt to: <user@xyz.com>)
user@abc.com replies (mail from: <user@abc.com>)

Now you want mail from: <user@abc.com> to be rewritten to mail from: <user@xyz.com>
However, when the original mail isn't forwarded from the xyz.com domain, no rewrite should take place. Correct?

In that case I guess it's pretty much impossible. The only somewhat feasable, yet complicated way, I can think of is to use the message-ID given by your smtp server to build a database of seen messages and then hope to catch as many replies as possible using the "In-Reply-To" header field. This, of course, relies on the fact that a) all email clients your users use actually write this field and b) your users do in fact hit reply rather than composing a new message.