sendmail, 4th Edition

FEATURE(relay_based_on_MX)

When sendmail receives a message bound for another host, it might be doing so because the local machine is listed as an MX record for that other host (How sendmail Uses DNS on page 325) and the other host is temporarily down. When that other host comes back up, sendmail will deliver all such queued messages to it. You allow relaying for hosts for which your site serves as an MX record, by listing the names of those sites in your relay-domains file ($=R on page 874).

There might be times, however, when your site must be an MX server for an unknown number of sites, or an unknown variety of domains. One such example might occur when your machine is behind a firewall on a private network. You might be the central MX site for all internal domains that are created or renamed often. For such sites, sendmail offers the FEATURE(relay_based_on_MX) that looks like this:

FEATURE(`relay_based_on_MX')

When you declare this feature, you allow sendmail to relay mail to any host for which your site is listed as an MX record. Fortunately, you don’t have to keep track of which hosts do list your site because this feature makes the process automatic.

This feature should be used only in an environment where you administer or trust the DNS records. You should not use it if your DNS lookups come from the Internet at large because, in that instance, anyone in the world would be able to use your machine as an MX server without your knowledge or permission.

Note that you should not ...