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
The form for the bestmx_is_local feature is:
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.