If you read the readme, it clearly covers this, it delivers to Maildir or blob storage like S3, your choice.

Clearly? I searched for maildir and mbox, my browser can't find either on that page. What does cloud storage have to do with the MDA?

Storage options: https://stalw.art/docs/get-started#supported-blob-stores On the first glance it seems that it's designed in a easily extensible way, so new storage options shouldn't be hard to add (https://github.com/stalwartlabs/mail-server/blob/main/crates...)

Given it's all-in-one thing, MDA/LDA is integrated, to best of my (very shallow, just ~30 minutes haphazardly checking docs and source code) understanding there is no separate delivery agent program sitting in-between the components, it's all a single process, but it has places where you can hook arbitrary external filters/transformers (https://stalw.art/docs/smtp/inbound/data#content-filters)

It is not directly in the README but there is a link in it to a getting started guide [0] which covers this. The user "zie" probably got it confused with the README.

[0] https://stalw.art/docs/get-started/