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 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:
(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.