For that particular example it should be pointed out that OpenBSD chroots the web server by default which ends up running as a non-privileged user. OpenSMTP does as much work as possible in multiple tasks running as a non-privileged user. So even ignoring the access control provided by the pledge system it is really unlikely that anything is going to escape to root or even be able to affect each other.

Sorry, I learned this lesson a long time ago. One service per machine. Especially this day and age where that's easy and cost-effective.