Document information

Document ID:4615
Subject:Preventing message loops in SMTP server
Creation date:12/14/15 4:35 PM
Last modified on:12/14/15 4:35 PM


Understanding and preventing message loops


Email messages, by design, hop from one SMTP server to another. Often misconfiguration in the one or more server can cause messages to get stuck in a loop - meaning server A sends a message to server B, which sends it back to A.

The following section describes how message loops are created in Xeams and ways to prevent them

Scenario 1


You are using Xeams as Hybrid-mode and the forward-to IP address is either missing or is defined as the IP address of Xeams itself. Refer to the image below.


Analysis

In Hybrid mode, Xeams forwards messages to another server. When configuring local domains, you need to specify the IP address of the actual server where messages should be forwarded.

This Forward-to IP Address goes into a local DNS file. Xeams does not contact a DNS server for MX lookup if the domain/IP is found in this local DNS file.

When a forward-to IP is missing or is specified as the machine where Xeams is running, messages will loop back to Xeams creating a loop.

Steps to prevent this situation

Ensure the forward-to IP address is correctly specified in local domain configuration


Scenario 2


You are using Xeams in Firewall mode and are using SMTP Proxy server. Besides In-bound messages, you want to use Xeams to handle out-bound messages as well. You configure your actual SMTP server to forward out-bound messages to Xeams.

Analysis

If your actual SMTP server forwards all out-bound traffic to Xeams' SMTP Proxy server, it will send the message back to your actual server, creating a message loop.

Steps to prevent this situation

If you need Xeams to handle out-bound messages, use the regular SMTP server rather than Xeams. Following steps show how to do this.
  • Assign a new IP address to the machine where Xeams is running
  • Configure Xeams SMTP Server to listen on this IP address. Once this is done, the SMTP Proxy server will handle in-bound emails and SMTP Server will handle out-bound messages
  • Configure your actual email server to forward messages to the new IP address you just added to your Xeams box.







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