Description
When attempting to send emails via the mailRelay service your mail server returns an NDR with '500 Firewall Error' in the body.
Solution
This particular NDR is typically encountered when a Cisco firewall protects your network and performs SMTP inspection. Either a compatibility or misconfiguration issue is the root cause.
To resolve the issue you can perform the following operations on your Cisco device:
- Login to your Cisco device
- Run show run to identify the ESMTP / SMTP inspection rule.
- Disable the ESMTP / SMTP inspection entry in the Cisco firewall configuration by inserting the word "no" at the beginning of the line, as shown in the examples below:
- no ip inspect name EXAMPLE123 es
- No ip inspect name EXAMPLE123 smtp
Warning
Modifying the configuration of your Cisco device is an advanced topic and should therefore only be undertaken by appropriately trained resources. The suggested solution is provided out of courtesy based upon previous feedback but should be tested before applying in your environment. As such the solution is provided as-is, without any warranty of any kind.