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Old 01-22-2007, 08:28 AM   #1
Registered: Jun 2003
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Postfix Mail Restrctions and Exchange Servers

Ok I apologize if this post is a little vague. Right now I don't have any of the bounce back messages with me to give a specific example of what I'm about to ask.

I've been running a Postfix mail server for about 10 months now. The thing works great and "it" never seems to have a problem. It also never seems to have a problem receiving mail from other Linux mail servers.

I'm using a lot of the Postfix client/sender restrictions. ie: reject_unknown_hostname, reject_non_fqdn, etc. This seems to cause a lot of problems when someone tries to send me an e-mail from an Exchange server.

I've never configured Exchange, and I get the feeling that I'm having issues receiving e-mails from Exchange servers with, well, not so knowledgable administrators. The issue seems to be that the Exchange servers' hostnames don't have DNS entries.

For example, let's say is being hosted on an Exchange server. The Exchange server's hostname may not be, it may be Now will more than likely have an A record in the company's DNS, however, if they do not include a DNS record for, then my Postfix server will reject the message. The Exchange sender will get a bounce back saying that hostname was unknown.

So is this my fault? Should I just remove the reject_unknown_hostname from my Postfix restrictions? Or is it the job of the Exchange administrator to make sure that his/her server has a valid DNS entry so it doesn't get flagged as spam?

The other issue seems to be that not many mail servers seem to use tight restrictions. So when I bring this up with the Exchange admin he just says "Well we don't have any problems sending to other e-mail addresses, the problem must be with your Postfix server."

Old 01-22-2007, 10:30 PM   #2
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So is this my fault? Should I just remove the reject_unknown_hostname from my Postfix restrictions?
You might consider using reject_non_fqdn_hostname instead. There's a link to this page on the postfix site. Here's the relevant part:
Q7. Why don't you use reject_unknown_hostname or

A7. Too many "false positives" (that is: rejects too much non-spam
email), in my experience.

(By the way, Derrick 'dman' Hudson brought up a very
good point in the postfix-users mailing list: If you're
going to use reject_unknown_hostname anyway, you
probably want to put it *after* reject_non_fqdn_hostname
to prevent an unqualified hostname from matching one in
your own domain.)
This point of view is expressed in many places, as you can verify with a google search.


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