Sendmail’s DNSBL lets users change the configuration file to enable the DNS Blacklist feature, which prevents receiving emails from IP addresses that are included on one or more blacklists. At Bobcares, with our Server Management Services, we can handle your server-related issues.

DNSBL In Sendmail

The dnsbl (DNS Blacklist) will block incoming emails from IP addresses on one or more blacklists. The way DNSBL functions are as follows:

1. Establishing a connection to the user’s server by a mail server that needs to transmit mail to it.

2. To determine the IP address of the server on the other end of the connection, the user’s server looks at the socket information.

3. The user’s server generates a unique “pseudo-hostname” by flipping the order of the IP address’s components and joining them with the blacklist’s domain name.

4. The user’s server performs a routine DNS query for the “pseudo-hostname”. If it succeeds, the blacklist’s owner believes the host to be a spammer. If the lookup is unsuccessful, the IP address is not on any blacklists.

How To Enable DNSBL In Sendmail?

1. Select a blacklist to use.

2. Check the mail log after adding a new blacklist to the setup and look for “check_.”

3. The sendmail.cf files are created from macro configuration files. Include a dnsbl FEATURE line for each blacklist in order to add a DNS Blacklist to the macro configuration file.

FEATURE(`dnsbl',`name-of-blacklist')dnl

How To Enable DNSBL Only For Specific Users?

For a number of reasons, DNS Blacklisting should only be made available to a portion of users. We may set sendmail up to only allow specified recipients to use DNS Blacklisting using the below steps:

1. Insert a special delay_checks feature line in the sendmail mc file, after the feature that enables the access database.

2. Add a line to the access db for each local recipient that wants DNS Blacklisting.

3. Regenerate the sendmail.cf and access.db.

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Conclusion

The article provides a two-step method to enable DNSBL in Sendmail. We also discuss the steps to allow DNSBL only for specific users.