100% on this suggestion.

i use to setup qmail to host a few domains, works really well, even mail blast is like really efficient. picked up a lot of fundamental about email, dns, ssl along the way.

just make sure you put a good filtering system before the email reaches your server. like mimecast, proofpoint, etc.

nowadays, you can further secure your access to pop3,imap on email server using service like cloudflare tunnel.