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How to configure the embedded Comprehensive Anti-Spam Service - CASS - with Load Balancing
03/26/2020 
7 People found this article helpful
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Description
When there are multiple public IPs for an email server (so multiple MX records), the inbound emails may be blocked by the CASS because they are processed incorrectly.
If there are multiple gateways, you may need to choose only one connection to enable the CASS, as you cannot use more than one public IP Address to process Anti-Spam Inbound traffic.
Cause
If you use the basic fail-over you can pick one connection as primary and enable the CASS only for that public IP.
If you use other fail-over methods you need to take into consideration that only one connection can use the CASS.
If for example other than the use of the secondary WAN link for fail-over, you would like to use it to balance the traffic with 50% ratio, you may want to use both links for inbound email traffic.
NOTE: You cannot configure the Anti-Spam Service with two public IPs for the email server, if you need to do that, you may need to use a dedicated Email Security solution.
You can however implement the following scenario if required:
Use both Internet connections at the same time to receive and send email from your email server using both links, keeping the Anti-Spam and Junk Store Services working for only one public IP.
Resolution
The embedded Anti-spam feature of the firewall allows you to specify only one Public IP address of the email server.
If you want to use a secondary WAN connection and you want to be able to receive the email on the secondary connection, you need to create a new Nat for a secondary public IP for the email server.
When you create the access rule for the inbound email traffic:
- Select the SMTP (Anti-Spam Inbound Port) service, only for the public IP configured under Anti-Spam Settings.
- For the secondary IP you need to specify SMTP (Send E-Mail) as the service.
CAUTION: The email going to the secondary public IP will not be filtered by the embedded Anti-Spam, so it is recommended that you only use this address as a secondary IP in case the primary ISP link goes down.
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