Before Stalwart, I used Rspamd. Rspamd had a history privacy feature that hashed (blake2 with a static prefix) the subject so you could not really see what was being emailed/logged in the interface. However, it was super handy to see some trends and understand specific emails without actually reading subjects.
I have noticed that Stalwart is pretty verbose and clear in its logs and UI. Obviously, this is a great thing! However, when I am doing maintainance or debugging certain issues, I don’t want to see what other users are doing. Whom they are sending to, or from who they are receiving email.
Hence I thought to add a privacy filter. This can go two ways, either very light light, just on the web interface and it blurs, hides, hashes, or whatever client side all email address (PII data). Or you can go more heavy, and actually “hide” it on the server side so that it also does not end up in logs etc.
Obviously, the question is what do you want to protect and from whom. So for me, having a toggle in the interface to (temporary) hide PII data in logs, queues, etc would be sufficient.
P.S. I can imagine that privacy focused folks would love to completely and irrevocably avoid logging/showing/storing PII as much as possible. However, that is a completely different path.