The class $=w ($=w) defines which hostnames will be treated as being equivalent to the local hostname. That method, however, requires that the mail administrator manually keep the class up to date.

As an alternative, for low- to medium-volume sites, use the bestmx_is_local feature. When enabled, this feature looks up each hostname that it finds in the bestmx internal database map (bestmx). That map returns the best MX record (if it is known) for that name. That returned record is then compared to the list of hostnames in class $=w to see whether it is equivalent to the local host. If so, the address is accepted for local delivery.

The form for the bestmx_is_local feature is:

FEATURE(`bestmx_is_local')

If you wish to limit lookups to a small list of domains, you can add them as a second argument:

FEATURE(`bestmx_is_local', `domain1 domain2 etc.')

Only the hosts listed are allowed to list your site as the best MX record for use with this feature.

Use of this feature is best limited to low-volume sites. Looking up every address in the bestmx map can cause numerous DNS enquiries. At high-volume sites the magnitude of extra DNS enquiries can adversely tax the system and network.

There is also a risk to this feature. Someone could create an MX record for your site without your knowledge. Bogus mail might then be accepted at your site without your permission:

bogus.site.com.    IN MX 0 your.real.domain

Here, mail to bogus.site.com would be sent to your site, where the name bogus.site.com would be looked up with the bestmx_is_local feature. Your sendmail would find itself listed as MX for bogus.site.com and so would accept the bogus mail and attempt to deliver it locally. If the bogus name were designed to discredit you, it could be set to sex.bogus.site.com, for example, and mail to root@sex would be delivered to you without you knowing the reason.