Postfix's architecture is the classic UNIX style of "do one thing and do it well", of composable processes.
So it's not a monolith, it's a composite. If that makes sense.
Sure, the components of postfix are not standalone tools, they are very much like systemd. One codebase (monorepo, yaay, old is new), sharing a lot of common internal code.
In the end multi-process composite (like oracle and postgresql RDBMSes) or a multi-threaded monolith (like MySQL or anything that runs on a JVM) is not that big of a difference nowadays. Both can be performant, both can be maintained well/efficiently by big teams and by a one-man-army (see how postfix is mostly maintained by Wietse Venema).
So it's not a monolith, it's a composite. If that makes sense.
Sure, the components of postfix are not standalone tools, they are very much like systemd. One codebase (monorepo, yaay, old is new), sharing a lot of common internal code.
In the end multi-process composite (like oracle and postgresql RDBMSes) or a multi-threaded monolith (like MySQL or anything that runs on a JVM) is not that big of a difference nowadays. Both can be performant, both can be maintained well/efficiently by big teams and by a one-man-army (see how postfix is mostly maintained by Wietse Venema).