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

   DSesmtp:[smarthost1]:[smarthost2]

as smart host definition in your sendmail.cf. 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:

   DSesmtp:smarthost

(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.