Very interesting project and impressive work, thanks for sharing!

Can you talk a bit about how data is replicated between nodes when Stalwart is run in clustered mode, and what kind of data integrity/resilience properties we have when one, two, several nodes go down?

Also, have you considered implementing server-side encryption of e-mail messages so that a "honest but curious" system administrators could not read user's messages? (e.g. using the user's password to derive an encryption key). More generally, what are your thoughts on the "privacy" aspect?