Kerio Connect can ask online blacklists if a sender's outgoing mail server is listed as a known spam offender. These blacklists greatly reduce spam, however they sometimes reject legitimate email. Kerio Connect allows you to define a custom whitelist of IP addresses to be excluded from the blacklist lookups. Some domains such as hotmail.com or yahoo.com mistakenly end up in blacklists, and there isn't an easy way to exclude all of their IP addresses.
Find out which blacklist is responsible
Kerio Connect reports all rejected emails and connections to the security log. If Kerio Connect is rejecting a message from a legitimate address, it will be reported in this log. You can search this log by right clicking in the window and choose 'find'. Perform an upward search from the bottom of the log for the sender's email address. The matched result will tell you the particular blacklist where this user's outgoing mail server was listed.
Change the action of the blacklist
Once you have identified the blacklist, you will need to change its action. From the Administration console, go to Configuration > Content Filter > Spam Filter. Under the Blacklists tab, edit the appropriate blacklist and change the action from 'Block the message' to 'Add spam score to the message'. Assign it a value of 10, which will ensure that any message from senders in this blacklist will be marked as spam, and can be filtered accordingly. Now that these messages are going to be received by Kerio Connect they can be processed by the spam filters, which will allow you to make exceptions based on header information.
Create a spam filter exclusion rule
Spam filter rules are defined in the Spam Rating tab under Configuration > Content Filter > Spam Filter. Create a new rule where the message header 'From' contains the email address of the sender who is being rejected by the blacklist. Choose to 'Treat the message as non-spam'.
