sendmail, 4th Edition

FEATURE(relay_hosts_only)

Normally, relaying is based on the domains listed with the RELAY_DOMAIN mc macro (The RELAY_DOMAIN mc macro on page 269) or in the file specified by the RELAY_DOMAIN_FILE mc macro (The RELAY_DOMAIN_FILE mc macro on page 269) or on the domains allowed to relay in the access database. When sendmail checks to see whether a domain should be allowed to relay, it interprets each domain as a top-level domain. For example, if RELAY_DOMAIN listed the following entry, or if the RELAY_DOMAIN_FILE file contained the following entry:

your.domain

all the following domains would also match that single domain entry:

sub.your.domain
a.very.deep.sub.your.domain

As an alternative, you can have sendmail interpret each name as the literal name of a host. If you prefer this second method, you can enable it by declaring the relay_hosts_only feature like this:

FEATURE(relay_hosts_only)

With this feature declared, sendmail will compare the sending host to the list of hosts, and to hosts looked up in the access database, on a host-by-host basis. For example, if the RELAY_DOMAIN defined the following:

sub.domain

only a host named sub.domain would be allowed to relay. Another host—say, hostB.sub.domain—would not be allowed to relay unless it too was listed, or OK’d by the access database.

Clearly this feature gives you more control over who can and cannot relay. It can be of value at a site that is populated by some network printers and some Unix machines. The file specified by RELAY_DOMAIN_FILE ...