Add postfix monitoring
Looks like we're pretty blind to postfix issues, which can be a problem when there's delivery issues, for instance for Add Forge Now inbound email.
https://github.com/sergeymakinen/postfix_exporter is a plausible candidate (looks more maintained and the metrics more idiomatic than https://github.com/kumina/postfix_exporter - which is unfortunately the one packaged in Debian)