exim problem with local usernames vs. ISP-mail name

[klaus_drews@ngi.de]

Hi,

I have a problem confuguring exim.
When I write mails that are delivered over my ISP, they have
my local username and local hostname as return-path, reply
and s an.
I fixed that with setting qualify_domain and qualify_recipient
to my local hostname and the domainname of my ISP / Mailprovider.
I then setup a user, which has the same name as my mailuser at
my provider has. Now that works only for one user with one 
mailaddress, not for different users.
I looked trough the FAQ and the webpage for exim, but could not
find anything, that works for me (or worked, when I tried it).
So what I want is, if user peter send an outgoing message, it
should be converted so as if peter_b@some_isp would have sent it.

Has anyone an example how to do this ? I think this is nothing
unusual - but I am quit new to exim an the rest worked fine.

KD.