To use FEATURE(mailertable), you will have to create an external database containing the routing information for various domains. For example, a mailertable file in text format might be:

	.my.domain		xnet:%1.my.domain
	uuhost1.my.domain	suucp:uuhost1
	.bitnet			smtp:relay.bit.net

This should normally be stored in /etc/mailertable. The actual database version of the mailertable is built using:

	makemap hash /etc/mailertable.db < /etc/mailertable

The semantics are simple. Any LHS entry that does not begin with a dot matches the full host name indicated. LHS entries beginning with a dot match anything ending with that domain name -- that is, they can be thought of as having a leading "*" wildcard. Matching is done in order of most-to-least qualified -- for example, even though ".my.domain" is listed first in the above example, an entry of "uuhost1.my.domain" will match the second entry since it is more explicit.

The RHS should always be a "mailer:host" pair. The mailer is the configuration name of a mailer (that is, an `M' line in the sendmail.cf file). The "host" will be the hostname passed to that mailer. In domain-based matches (that is, those with leading dots) the "%1" may be used to interpolate the wildcarded part of the host name. For example, the first line above sends everything addressed to "anything.my.domain" to that same host name, but using the (presumably experimental) xnet mailer.

In some cases you may want to temporarily turn off MX records, particularly on gateways. For example, you may want to MX everything in a domain to one machine that then forwards it directly. To do this, you might use the DNS configuration:

	*.domain.	IN	MX	0	relay.machine

and on relay.machine use the mailertable:

	.domain		smtp:[gateway.domain]

The [square brackets] turn off MX records for this host only. If you didn't do this, the mailertable would use the MX record again, which would give you an MX loop.