The FEATURE(dnsbl) is used to enable the blocking of email from open relay sites, dial-up sites, or known spamming sites. It does so by invoking the rbl technique discussed in the previous section. The feature is included in your mc configuration file like this:

FEATURE(dnsbl)                           ← simple form
FEATURE(dnsbl, `optional arguments')declared with arguments

In its simplest form, when mail arrives from a site, that site’s IP address is reversed and prefixed to the default host blackholes.mail-abuse.org.[125] If the lookup succeeds, the host is considered bad and the following error is sent in reply to the initial connection:

550 5.7.1 Rejected: IP listed at blackholes.mail-abuse.org

If the address is not found, the connection is allowed and the mail is accepted depending on subsequent SMTP and header checks. By default, temporary failures are ignored and the connection is treated as good. If you wish temporary failures to cause the sending site to defer the message, you can supply a third argument such as this:

FEATURE(dnsbl, , ,`t')

If the third argument is a literal t character, instead of ignoring temporary errors, the following will be returned:

451 Temporary lookup failure of IP at blackholes.mail-abuse.org

An argument can be supplied to this feature if you wish to use a lookup host other than, or in addition to, blackholes.mail-abuse.org. The canonical name of the lookup host is simply inserted following a comma after the literal dnsbl:

FEATURE(dnsbl,`dialups.mail-abuse.org') ...

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