[exim] rejecting mail to non-existent hosts

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Author: Zbigniew Szalbot
Date:  
To: exim-users
Subject: [exim] rejecting mail to non-existent hosts
Hello,

I am trying to understand why my exim-4.66 does not outright reject
emails destined to non-exisent users. Can you please help me by
suggesting which parts of the configuration I should check?

Here is an example session with a faked nokia.com address.

2007-04-27 10:55:06 1HhMCs-0007G8-R6 <= a_rutgers@???
H=(smtp-in.wanadoo.nl) [196.202.121.201] P=esmtp S=15658 from
<a_rutgers@???> for bf7122510839@???
2007-04-27 10:55:06 1HhMCs-0007G8-R6 ** bf7122510839@???:
Unknown user
2007-04-27 10:55:06 1HhMEM-0007IF-Bm <= <> R=1HhMCs-0007G8-R6 U=mailnull
P=local S=16503 from <> for a_rutgers@???
2007-04-27 10:55:06 1HhMCs-0007G8-R6 Completed

What bothers me is that normally any email sent to a non-existent user
is rejected and not bounced. Why does exim bounce it? For example:

I tried sending an email from another host to lists.lc-words.com domain
and the mail came back to me like this:

<glupiaala@???>: host lists.lc-words.com[83.19.156.210] said:
    550 Unknown user (in reply to RCPT TO command)


Exim recorded it in its log:
2007-04-27 18:06:52 H=max.cyfronet.com [85.14.85.15]
F=<SOMEONE@???> rejected RCPT <glupiaala@???>:
Unknown user
2007-04-27 18:06:52 SMTP connection from max.cyfronet.com [85.14.85.15]
closed by QUIT

But in the faked nokia case the email seems to be bounced, nokia does
not accept it and it gets frozen. I may be wrong with the interpretation
of the logs. I will be grateful for your suggestions.

Thank you very much!

Zbigniew Szalbot