[exim] Exim/Postfix once again

Good day folks,

I am unsure whether this question belongs here or to Fetchmail.
I am quite sure though, that it might have already been discussed.
I wasnt able to find anything helpful in the archives so please
bear with me if this is a FAQ.

A user is using a multidrop box for his domain name. He picks this
up using fetchmail and distributes it to several users. He is
using the Envelope-To header to detect the recipients of the
single e-mail and distributes it among local users at his server.

In case an e-mail has multiple recipients, Exim does not add the
"for" address in the Received header so he needs to rely on those
Envelope-To header, which might be better anyway. He is now complaining
that user's addresses who received the e-mail as a Bcc are leaked
to other recipients because of the Envelope-To header.

The question if it is possible to strip the envelope-to header after
analysing it might be a question for the Fetchmail folks (I would be
happy about a hint anyway). Fetchmail manual however suggests that
the MTA stores one e-mail per recipient in the box although it is
actually the same box and adding proper Envelope-To or X-Envelope-To
headers.

Can someone help me with a hint/best practice scenario how to achive
this? I have searched the Exim-docs over and over again but was
not able to find any information (not saying that it is not there ;-)).

Thank you
Sascha