[afnog] is my sendmail an open-relay?

[Brian Candler]

On Tue, Nov 07, 2006 at 07:54:10AM -0800, John Walubengo wrote:
> how do i ensure my mail server is not acting as an open
> relay? Am runing sendmail 8.12.x on RH9. I wish to have
> sendmail recieving ONLY mail destined to my domain and
> sending out ONLY mail originating from my private network.
> 
> which files do I need to tweak/review?

Whilst it's good to check the configs, the final assurance comes from doing
a real test.

Get an account on a machine on a different network than yours (or get
someone else to perform this test). On that machine, type

  telnet your.mail.server 25

It should connect and give an ESMTP banner. Try to submit a mail by hand:

  EHLO testing
  MAIL FROM:<you at yourdomain.com>
  RCPT TO:<other at otherdomain.com>

You should get back a "550 Relaying denied" or similar error at this point.

Regards,

Brian.