Overview

You wish to know how a specific type of file, e.g. htm, html, zip archive etc, can be blocked from inbound and/or outbound emails. You could be looking for this information because you suspect some of these extensions could contain malware.

Solution

Kerio Connect has a built-in file filter feature that allows detecting a specific type of file and automatically block and filter malicious attachments. Please note the following before you proceed with the solution steps:

• Depending on Attachment Filter rules, the specific file or MIME type extension can be blocked or accepted. 
• Filter rules are applied globally for all domain users from incoming and outgoing emails - hence these rules will apply to all users, and will affect both sent and received attachments. Currently there is no way of making exceptions.
  • You can set up an address to receive all blocked attachments in case you might need to access the blocked attachments, as shown in steps below
• The filter only checks the file's extension and doesn't scan the file to make sure if the file's name (and extension) really match up with the file's content.

Enabling Attachment Filter

1. Log in to the Kerio Connect administration console.
2. Navigate to Configuration > Content Filter > Attachment Filter. 
   
   
   
   
3. Check the Enable attachment filter option.
4. Mark the desired filter options.
   1. If you want Kerio Connect to notify senders that their attachments were not delivered, select the option Send the sender a warning informing that the attachment was not delivered.
   2. To have Kerio Connect send the original messages to a different email address, select the option Forward the original messages to and enter the required email address.
   3. To have Kerio Connect send the filtered messages to a different email address, select the option Forward the filtered messages to and enter the required email address.
   4. To discard the ZIP attachments with dangerous files, select the Discard zip archive containing files with dangerous extensions option. 
      
      
5. Enable any of the predefined filter rules. Each rule can allow or block a specific file type.
6. Click Apply.

Creating Custom Attachment Filter Rules

1. Log in to the Kerio Connect administration console.
2. Navigate to Configuration > Content Filter > Attachment Filter.
3. Click Add.
4. Enter a description of the new rule.
5. Define the Condition for the attachments - Filename or MIME type
   
   
6. Select/enter a file type. The field is editable.
7. Select whether Kerio Connect blocks or accepts messages with this file type.
8. Click OK.