As an anecdote – I have been sitting on an elastic IP at AWS for years, with reverse DNS configured properly for it. Way early on (years ago), some spam filters would block the whole netblock, but I can’t remember the last time the IP Block was wholesale blocked. I think AWS is very much on top of any spam complaints from their Elastic IPs, and as long as you don’t abuse your specific IP, you are in good shape for light volume, non-spam mail.
LMTP support would be nice too: existing mail routing infrastructure could send messages into stalwart-managed mailboxes. (Edit: reading the docs, they do support LMTP! This is awesome)
A single binary can be invoked with different privilege levels. OpenSSH, for example is a single binary, but uses OS privilege separation when setting up connections from the root-owned daemon. (Just to be clear, I’m not sure that stalwart is using this technique, just that single binary apps do not exclude the possibility of OS privilege separation.)
As an anecdote – I have been sitting on an elastic IP at AWS for years, with reverse DNS configured properly for it. Way early on (years ago), some spam filters would block the whole netblock, but I can’t remember the last time the IP Block was wholesale blocked. I think AWS is very much on top of any spam complaints from their Elastic IPs, and as long as you don’t abuse your specific IP, you are in good shape for light volume, non-spam mail.