The features are toggles or allow you to slot in your own infrastructure where need be, from what I can see. It's not that you have to use it, it's that it's there if you want it, and I want it.
Suits my needs, but I can see why it wouldn't suit everyone's.
But the messages are accessible by every open standard: POP, IMAP, JMAP. You can also pick your storage backend, your database backend, your full text/search tool or provider, directory backend. It is amazingly versatile.
Between all the options, you can design incremental backups, snapshots, or whatever with 3rd party tools to write a script to backup your mailboxes to be restorable in any other email service or software. I have tested it with rsync, restic, database dumps, mc/aws-cli depending on the backends used, of which I have tried them all, and found it designed to be very straightforward.
The monolithic aspect is a necessary aspect of being built for HA and distributed environments that it is all the more impressive how versatile it is.
The only way to adopt Stalward is to drop everything else and use a single monolithic do-it-all?
Messages are stored in a bespoke format and not easily accessible directly?
It doesn’t sound like it’s made to be usable with other software. This isn’t an advantage in my book.