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I am in the process of setting up a mail server (using sendmail to be exact). The server needs to be as open towards receiving notifications of "bad"/bouncing emails as entirely possible.

I am not exactly into the nitty gritty details of how this entire bouncing/bad email thing works, so I am not sure exactly how much I need to do to make this work.

Currently if I try to send an email to an email address on a domain hosted within Google Apps I get a notification in return just fine. However as there's no ports open on the server sending the emails out, I am thinking there could be some problems if the email is bounced further out the process?

Does anyone know how to best achieve this? Is this as simple as opening a port? Or are there other things that needs to be taken into consideration?

Any pointers / links to resources would be greatly appreciated. I am not a system administrator - but unfortunately the company I work for do not have any, so someone has to play the role.

EDIT: As stated, the type of system needed to be expanded on: The server is a debian lenny system, sendmail is installed through the official repositories (through "apt-get install...") - everything else is left to the stock configs.

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Yes, you will need to make sure that your sendmail daemon is listening on port 25. The method for doing this differs from unix flavor to unix flavor, so if you can provide that info that will be info that helps provide a more detailed answer.

Apart from that, you'll want to make sure that you don't have any firewalls (or iptables rules) that block port 25 on your mailserver.

With the additional info that you've added, I can answer the question: You should have a file in /etc/mail called -- edit that in your favorite editor and look for these lines:

dnl # Daemon options - restrict to servicing LOCALHOST ONLY !!!
dnl # Remove `, Addr=' clauses to receive from any interface
dnl # If you want to support IPv6, switch the commented/uncommentd lines
dnl #
dnl DAEMON_OPTIONS(`Family=inet6, Name=MTA-v6, Port=smtp, Addr=::1')dnl
DAEMON_OPTIONS(`Family=inet,  Name=MTA-v4, Port=smtp, Addr=')dnl

On the very last line you're going to take out the "Addr=" (and the preceding comma), and then restart your sendmail process (via /etc/init.d/sendmail restart). (This will cause your /etc/mail/ to be recreated from the that you just edited.)

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I added the additional system information in the original question. Thanks for taking the time to answer the question :) – kastermester Sep 14 '10 at 19:30

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