Avoiding IP Reputation Problems with Redundant Mail Paths

Some organizations wish to deploy complete end to end redundancy for their Exchange environment, including the outgoing routes to the internet.

To achieve this most organizations will simply provision a backup internet connection for their network.  This connection can either be activated during an outage of their primary link, or be configured as a secondary route that will be automatically used if the primary route is down.

Although this seems like a simple win it can cause problems with email delivery because of IP reputation issues.

You need to be sending email fairly consistently from an IP address in order to maintain a decent reputation for that mail source. If you treat a second location as a cold standby, only used when your main ISP breaks, expect to see serious delivery problems as you migrate across to it.

In other words unless you are continually sending email out both of your email routes you might create new problems for yourself when you start using the backup connection.  So what is the solution?

Better to spread load across both locations, to keep both sets of addresses “warm”

Load Balancing Outgoing Email with Exchange Server

A common misconception is that outbound email can be load-balanced for Exchange simply by provisioning two equal cost Send Connectors, either using DNS to route directly or routing via a smart host for each Send Connector.

However this is not true.

If multiple equal cost connectors are available to route email, E2007 Routing picks one of the connectors deterministically… Mail will not be load balanced among multiple equal cost connectors.

When the cost of the Send Connectors and the proximity to their source servers are the same, Exchange will simply choose the one with the alphanumerically lower connector name, and will not load balance the outgoing email across both connections.

The correct solution is to deploy a single Send Connector with multiple smart hosts.

If the smart hosts are on your own network then they are configured to route to the internet via their respective ISP connection.

Or if the smart hosts are actually hosted by the ISP then the Send Connector simply specifies the IP addresses or DNS names of the smart hosts, and the Exchange source servers would need static routes configured to be able to reach each smart host over the correct ISP connection.

When multiple smart hosts are configured on a single Send Connector the outgoing email will be correctly load balanced.

If a smart hosted SMTP Send Connector has multiple smart hosts defined, load balancing and fault tolerance are accomplished using these smart hosts.

In summary, to achieve outgoing email load balancing with Exchange Server 2007 and 2010 without creating delivery problems due to IP reputation:

  • Do not configure multiple equal cost Send Connectors
  • Do configure a single Send Connector with multiple smart hosts
Written by Paul Cunningham

Paul lives in Brisbane, Australia and works as a technical consultant for a national IT services provider, specialising in Microsoft Exchange Server and related messaging systems.


  1. Mahmoud · July 20, 2011


    But I have read some where, configuring multiple smart hosts in one send connector does not provide load balancing instead provide fault tollerance,; the send connector will keep using the first smart host specified and if it is busy or failed it will use the second one!! is that what you meant!


  2. Lonney · April 15, 2013

    “When multiple smart hosts are configured on a single Send Connector the outgoing email will be ***correctly load balanced.***” The link to the TechNet article confirms this is you take the time to read it :)

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