DNS blacklists (DNSBL)

Note: This feature requires a properly configured DNS server. If the DNS server is not properly configured (and we have seen this many times), a time out will occur and mail will be processed slowly and in addition a lot of valid mail will be tagged as spam. For more information see the GFI Knowledgebase article `KBID001770'.

GFI MailEssentials supports a number of DNS blacklists, which can be configured from the DNS blacklists node. DNS blacklists are databases of SMTP servers that have been used for spamming. These databases are queried ` DNS style'. There are quite few third party DNS blacklists available, ranging from reliable lists that have clearly outlined procedures for getting on or off the DNS blacklist to less reliable lists.

When an email is sent, it is passed through a number of SMTP servers until it reaches the final destination. The ip address of each of these SMTP servers is recorded in the email header. GFI MailEssentials will check all the public ip's found in the message header with the DNSBL database configured (example: ordb.org).

The ORDB list is an Open Relay Database maintained by ORDB.org. ORDB.org is a non-profit organization, which stores an IP-addresses of verified open SMTP relays. These relays are, or are likely to be, used as conduits for sending unsolicited bulk email, also known as spam. By accessing this list, system administrators are allowed to choose to accept or deny email exchange with servers at these addresses.

How it works

GFI MailEssentials will check all the public ip's found in the message header with the DNSBL database configured (example: ordb.org). GFI MailEssentials will record all the ip's checked in an internal database and will not perform further checks with the DNSBL for the same ip's. The ip addresses are kept in the database for 4 days, or until the Simple Mail Transport Protocol service is restarted

Right-click on the Anti Spam > DNS Blacklist node and select properties.

Click on `Check whether the sending mail server is on this DNSBL'.

Now select the appropriate DNS blacklist you wish to check incoming mail against. For example relays.ordb.org

Optionally you can select a second DNS blacklist, for example SpamHaus.

Note that querying a DNS blacklist can be slow (depending on your connection), so mail can be slowed down a little bit, especially if you query against 2 DNS blacklists.