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12. Juli 2019

Smarthost load balancing with Sendmail

Abgelegt unter: Hacker's Corner — Thomas Omerzu @ 13:47

If you have multiple smart hosts that can work as outgoing mail relays, you might wish to implement a kind of load balancing.

On the Internet, you often find the proposal to use something like


as smart host definition in your Unfortunately, for us this didn’t work: It implements only a failover, as Sendmail always uses the first host as long as it is reachable.

So we finally came to this solution:


(note the missing square brackets, which mean that an MX record lookup will take place). In the DNS, we then define

   smarthost  MX  42 smarthost1
              MX  42 smarthost2

And voilà: Sendmail automatically balances outgoing mails between those two MX hosts as they have equal priority.

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