Before creating any user accounts, create an acceptable use policy that, among many other things, gives you the power to block spam communications—both inbound and outbound. Ensure that all users agree to this policy before giving out any user accounts.

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The domain administrator must create a DNS zone file, in the proper format, that lists all of the connection addresses that are to be blacklisted. The special DNS address records in the zone file are constructed by reversing the IP address of the blacklisted system to create the DNS name field of the record and by using an address such as 127.0.0.2 in the data field of the address record. This format means that hosts are blacklisted by IP address instead of by name, which makes sense because the dnsbl lookup is done using the connection IP address. The DNS server must be authoritative for the domain in which the blackhole list is to be located. This is normally done by creating a special subdomain for the blackhole list within the zone of authority of the DNS server.

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Add the dnsbl feature to the sendmail configuration. Identify the specific blackhole list service you wish to use on the dnsbl command line. Here is an example:

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You use a blackhole list service that blacklists a few sites with which you must communicate. You need to configure sendmail to override the blackhole list for specific addresses.

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Create a minimal sendmail configuration containing only an OSTYPE statement to specify the correct operating system and a FEATURE command to select the nullclient feature:

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