We’ve covered setting up your sendmail to act as a relay for certain computers. Now, we look at another relaying problem.

You like running your own sendmail, you’re using it to manage your own email accounts. You could use your ISP’s mail server for all outbound messages, but let’s say you’re not doing that. Now, some third party, maybe another ISP, let’s call them “Dogers”, decides to silently discard all email coming from IP blocks owned by your ISP unless the sending IP number is one of the mail servers of your ISP. Even if you’re running a responsible sendmail on a static IP number, messages sent to “Dogers” just vanish.

The solution is to arrange your sendmail so that, when sending to certain domains, it relays the messages through your ISP’s servers. We’ll need two more features for this. First, the mailertable function will allow you to use a different mailer for certain addresses. Second, depending on your ISP, you may have to authenticate yourself with the ISP’s server before it will relay your messages. This configuration will show how to perform that authentication.

Make sure your sendmail.mc contains the following two lines before the first “MAILER” line:

FEATURE(`authinfo',`hash /etc/mail/auth/client-info')dnl
FEATURE(`mailertable')

Also, add the following line anywhere in the file:

define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl

You will need to have cyrus-sasl installed, and configured for logins. Here is a sample cyrus-sasl configuration invocation:

./configure --prefix=/usr/local --enable-anon --enable-plain \
       --enable-login --disable-krb4 --with-mysql \
       --with-saslauthd=/var/state/saslauthd --with-openssl=/usr/local/ssl \
       --with-plugindir=/usr/local/lib/sasl2/ --enable-cram \
       --enable-digest --enable-otp --without-des

OK, now the mailertable entry. Add a line for the dogers domain, telling your sendmail to forward mail for those addresses through your ISP’s server:

dogers.com      smtp:smtp..

Now, to authenticate with the ISP. We told sendmail that our credentials would be stored in /etc/mail/auth/client-info, so we create a file there:

AuthInfo:smtp.. "U:root" "I:wintertoad@." "P:" "M:LOGIN"

Then, we just have to rehash the mailertable and authentication files with a command like this:

# makemap hash file.db < file

Now, assuming you’ve rebuilt your sendmail.cf after the changes we made to the .mc file above, you can just send a SIGHUP to the sendmail processes, and you should be able to send email to anybody at the dogers.com domain by relaying those messages through your ISP’s mail server.