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allowing sendmail to relay mail
10 February 1999
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This article deals with allowing sendmail to accept mail for
specific domains, often referred to as relaying. If you have installed sendmail
8.9.3 or later, it will automatically disable relaying. In simple terms, mail relay
is a mechanism by which mail is transferred from host to host until it reaches its
ultimate destination. I use langille.org as the example domain. I've recently registered this domain and I'm about to tell sendmail about it. Note: More recent versions of FreeBSD put all sendmail configuration files in /etc/mail not /etc. Note: sendmail.cw has been renamed to local-host-names after sendmail version 8.10. |
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Why disable relay
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| sendmail 8.9.3 comes with relay disabled to prevent abuse of your
mail server. If relay were permitted, anyone could use your mail server to send out
mail. Spammers love mail servers which don't restrict relay. If they find one,
they send all the mail they can through it. Spammers do this because it uses someone
else's resources and partially masks their identity. By the way, spammers are people who send out junk mail. The basic reason you should disable relay is to first prevent the theft and abuse of your mail server resources. |
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Why enable relay
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| You want to enable relay only for those domains from which you wish to
send mail and for those domain for which you wish to receive mail. This doesn't mean
that you have to specify each domain which can send you mail. No, this is much
simpler than that. I just added a new domain to my collection, langille.org. At present, my mail server does not accept incoming mail for that domain. However, my mail is registered as the mail host for that domain. Here's how to verify that: # host langille.org langille.org mail is handled (pri=5) by freebsddiary.yi.org |
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Incoming mail - relaying denied
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If someone tries to send mail to langille.org, it will arrive at my
server, but it will be refused. Here is the bounced email message. The exact
message will vary from ISP to ISP:
Here are the corresponding messages in my mail log:
It is important to note that the above example represents mail being sent to langille.org from an external domain. It is incoming mail. To allow sendmail to receive mail for langille.org, I added the following entry to /etc/mail/sendmail.cw (after sendmail version 8.10, this file is local-host-names). Note that more recent versions of FreeBSD put all of these files in /etc/mail/. langille.org Then I told sendmail to re-read it's configuration files by issuing the following commands. # killall -hup sendmail With the above changes, my mail server began to accept incoming mail for langille.org. |
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Outgoing mail - relaying denied
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If you try to send mail through your own server and you get the following
message, then you need to modify /etc/mail/relay-domains.
From: Mail Delivery System <> To: Self Subject: Mail Delivery Failure. Date sent: Sat, 30 Oct 1999 15:14:10 Delivery has failed on the enclosed message for the following reasons reported either by the mail delivery system on the mail relay host or by the local TCP/IP transport module: 550 <junkmale@xtra.co.nz>... Relaying denied Your original mail message follows: -------------------------------------------------------- X-PM-Identity: langille.org From: "Dan Langille" <dan@langille.org> Organization: langille.org To: someone@some.other.domain Date: Sat, 30 Oct 1999 15:14:09 +1300 MIME-Version: 1.0 Content-type: text/plain; charset=US-ASCII Content-transfer-encoding: 7BIT Subject: test langille.org Reply-to: dan@langille.org Priority: normal X-mailer: Pegasus Mail for Win32 (v3.12a) -- Dan Langille http://www.langille.org The solution to this problem will be in your mail logs. Look at the following carefully:
You will see that the relay attempt came from the host starlajo.nz.freebsd.org with IP=10.0.0.45. So I added the following entry to /etc/mail/relay-domains:
Then I hup'd sendmail. |
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Summary
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| /etc/mail/relay-domains contains a list of hosts which are
allowed to relay mail through your mail server. This list may consist of either
specific hosts or whole domains. /etc/mail/sendmail.cw (after sendmail version 8.10, this file is local-host-names) contains a list of domains for which your mail server will accept mail. This list is usually the domains hosted by your machine. NOTE: put a CR/LF at the end of the last domain. Otherwise it won't work. |
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Silly but practical examples
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| Your friend's domain is retch.org. You wish to allow your friend to
use your mail server. The box he will be sending mail from is dry.retch.org. You would add the following entries to the files on your mail server:
The above tells your mail server to accept incoming mail for the domain retch.org (after sendmail version 8.10, this file is local-host-names). |
| I found, by trial and accident, that /etc/mail/sendmail.cw needs a CR/LF after the last domain. Otherwise sendmail doesn't pick up that domain. So make sure you press ENTER after the last entry in that file (after sendmail version 8.10, this file is local-host-names). |