MailEnable Enterprise Guide
SMTP - Reverse DNS blacklisting
Reverse DNS Blacklisting allows DNS based blacklists to be used with MailEnable. This can help to control spam.  It is possible to select which RBL blacklist providers to use, however, only the select providers that are needed as this feature has an impact on performance.

DNS blacklists are lists of IP addresses that are not allowed to connect to the email server. These lists are formed in various ways. Some lists are simple listings by country, some list known spammers and some are reactive and add entries only after an IP address was responsible for sending out junk email. Blacklists have a high risk of causing "false positives", which means that legitimate email may be refused. Before using DNS blacklists, it is wise to do some research on how the lists are maintained, what the removal process for listed IPs is and what their motivations and goals are with their list.

How to add a Reverse DNS blacklist for spam filtering

  1. Within the Administration program navigate to: Servers > localhost  > Connectors > SMTP
  2. Right click on SMTP and select properties in the menu.
  3. Select the DNS Blacklisting tab.
  4. Tick the option to Enable DNS Blacklisting
  5. Select the desired Action when detected (the default is Don’t accept the email).
  6. Click on the Add button to select a blacklist.
  7. Select a blacklist and then click OK.
  8. The selected blacklist will be displayed within the Selected DNS Blacklists display window.
  9. Repeat this process to enable multiple lists.

How to add a URL blacklist for spam filtering

  1. Within the Administration program navigate to: Servers > localhost  > Connectors > SMTP
  2. Right click on SMTP and select properties in the menu.
  3. Select the DNS Blacklisting tab.
  4. Tick the option to Enable URL Blacklisting
  5. Select the desired Action when detected (the default is Don’t accept the email).
  6. Click on the Add button to select a blacklist.
  7. Select a blacklist and then click OK.
  8. The selected blacklist will be displayed within the Selected URL Blacklists display window.
  9. Repeat this process to enable multiple lists.

How to configure custom blacklists

  1. Within the Administration program navigate to: Servers > localhost  > Connectors > SMTP
  2. Right click on SMTP and select properties in the menu.
  3. Select the DNS Blacklisting tab.
  4. Click on the Configure Blacklists... button.
  5. Click on the Add button.
  6. Next specify a blacklist name.
  7. In the Blacklists details section specify the lookup type and zone and the record type to check for.
  8. Next click Save.

DNS and URL blacklisting options

Setting

Explanation

Current Enabled DNS Blacklists

Shows all lists that have been enabled for the server.  This includes the MailEnable defaults and any personally created lists.

Add Button

To choose a blacklist, select this button, select a list and click OK. The list will now be displayed in the “Current enabled DNS Blacklists” window on the DNS Blacklisting TAB.

Remove Button

To remove a list at any time, select the blacklist in the “Current enabled DNS Blacklists” window on the DNS Blacklisting TAB and select the Remove button.

Enable DNS Blacklisting

Enables or disables reverse DNS Blacklisting for the SMTP Connector.

Action when detected

The two actions here are;

Don’t accept the email - this will prevent connection by the remote server and respond accordingly. This is the best option for reducing server load.

Mark the message as spam - by adding a line to the header.  If enabled the message will be delivered to the Junk E-mail folder within the email client.  For further information on the Mark Message as Spam action please review Feature selection in the Message store section.

Enable DNS Blacklitsing

When enabled all messages will have their content scanned for links to web sites. When a link is found, then MailEnable will check the IP addresses of any URLs found to see whether they are contained in the selected blacklist.

Enable URL Blacklisting When enabled will check URL's in the body of emails against DNS blacklists to determine weather it is spam.

Action when detected

The three actions here are;

Don’t accept the email - this will prevent connection by the remote server and respond accordingly.  This is the best option for reducing server load.

Mark the message as spam  - by adding a line to the email header indicating it is spam.  This will allow locally delivered messages to be delivered to the Junk E-mail folder. For further information on the Mark Message as Spam action please review the Feature selection section.

The “Replace the link” option will remove the failed link URL of the message and replace it with “Link is removed”.

Configure Blacklists Button

Opens a screen to allow blacklists to be created or added.

Lookup type

The lookup type that will be used for the blacklist.

Zone Server

The name of the DNS Zone or the IP Address of the DNS host that should be queried.

Record Type to check for

When the remote host or zone is queried, it may return one or more DNS Record types. Most implementations return an A record, but other implementations may return NS, PTR or MX records.

Response

The response that can be sent to the client when a message has been rejected.

Note: It is possible to configure a white list that will override the reverse DNS blacklist. This is configured in the administration program by selecting the White list button on the Reverse DNS Blacklisting tab under the properties of the SMTP Connector.
Note: Reverse DNS blacklists affect the performance of incoming email. The reason for this is that for each inbound connection, MailEnable will perform a lookup in the remote DNS.

 

 


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