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Seamless SMTP failover(and round robin) in outbound Domino connections

We still get comments on a blog post I did 9 years ago on SMTP Debug for Domino SMTP Routing. (I’ve updated non-working links today). This got me thinking on what other SMTP config would be useful to people.

Here I’m going to cover an often underused setting relating to Domino SMTP Routing.

If you’re sending mail outbound over SMTP from Domino, generally the default position that I tend to see is either:

  1. One hostname or IP is populated in the Configuration document->Basics-> Relay host for messages leaving the local internet domain (or Local Internet domain smart host). OR
  2. One hostname or IP is set in an outbound connection document. Sometimes there’ll be multiple connection documents to multiple Hostname/IPs.

The issue with Scenario 1 is it allows for minimal redundancy, (even if the hostname specified is a load balancer), if the hostname specified goes offline there’s no outbound SMTP from Domino.

The issue with Scenario 2 is again if only one connection document is in use, there’s minimal redundancy. If you have multiple connection documents, it’s quite clunky and you’ve limited control. i.e. you mightn’t want SMTP traffic to go over a backup line to a DR site unless it’s an emergency for example.

Well fear not, of course Domino does allow you more control over your outbound SMTP settings. You can set up either your configuration document (Router/SMTP->Basics->Relay host for messages leaving local internet domain) or your connection documents to use either round robin OR Failover on SMTP outbound connections.

“Failover and round robin sounds complicated what does this mean?”

It’s not really, what we are saying for failover is that you set your connection document/configuration document to have a list of IPs/hostnames. When an outbound SMTP connection is initiated – Try the connection to the first IP/hostname, if that doesn’t work, try the next, if that doesn’t work try the next.

For round robin, what we are saying for each connection, try the next one on the list (i.e. spread the load).

How do you turn these options on?

For Failover, simply add two or more IP addresses/hostnames separated by a semicolon with square brackets at the start and end and in between each IP address. The Connection Document/Configuration document will try the IP addresses/hostnames from left to right, the first time there’s a successful connection it stops.

Example of Failover is below

For Round Robin, simply add two or more IP addresses separated by a comma with square brackets at the start and end and in between each IP address. Each connection will connect to each IP in order. Like this:

If you are using hostnames, make sure the Domino server can resolve these correctly.

I hope you found this useful! Hopefully people will still be commenting on this blog post in another 9 years!!

Cormac McCarthy – Domino People Ltd

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