To create a Content Filtering rule for incoming emails:

1. Create a Special Group:
   1. Go to Administration > Special Groups.
   2. Click Add.
   3. Type a name for the special group. Use names such as "Internal" or "my domain".
   4. Under the Select Accounts drop-down menu, select Enter SMTP Address.
   5. Type "*@<your_domain>" and click Add >>.
   6. Click Save. 
2. Navigate to the Content Filtering screen.
3. Click Add > Match any or apply to all to add a new policy.
4. On the Step 1: Select Accounts window, do the following:
   1. Select From specific senders to any recipients for the policy scan.
   2. Click specific senders/specific recipients.
   3. Select Anyone on the Select Accounts screen.
   4. On the Exclude Accounts screen, choose Browse from special groups.
   5. Select the special group that you created earlier.
   6. Click Save and then click Next.
5. On the Step 2: Specify Policy window, do the following :
   1. Select Target according to your needs.
       

      Note: By default, Subject and Body are ticked.
   2. Add keywords you wish to apply this policy to, such as “invoice”.
   3. Click Next.
6. On the Step 3: Specify Actions window, do the following:
   1. Select desired action based on your requirements, such as Quarantine entire message.
   2. Click Next.
7. On the Step 4: Specify Notification window, configure the Notification Settings and click Next.
8. On the Step 5: Name and Priority window, do the following:
   1. Enable the policy.
   2. Supply a policy name and specify the priority of this policy.
   3. Click Save.