For what it’s worth, I have a server running for a couple of weeks with four domains and about two mailboxes per domain. It’s
low volume but it works on a 1GG RAM and 1CPU on a VPS. I have a 1.5G swap but use a third of it. I have
@accolon is right, you can find good cheap VPS, mine costs $5/month. Just make sure they let you host a mail server.
Hope this helps. I was discouraged with the minimal settings found on the mailcow website but it’s not that bad if you disable the above.
Best of luck.