Hey! I'd not noticed you were involved. EDIT: Where are my manners. Hope you're well too!

So, yeah, qmail is fine as a starting base, but there are important things that a modern MTA/MSA needs to have:

  - MSAs need to support user authentication in
    SUBMIT (no open relays)
  - MTAs and MSAs need to support TLS
  - MTAs should -really, need to- support DANE
    for authenticating SMTP servers

There's more stuff too (delivery options, filtering, ...), but protocol-wise, the above are critical.

I'm not sure that portability is such a problem that you can't use Rust. Rust runs on all modern OSes, which is all you need to support in a new MTA/MSA. And Rust is fairly easy to get into, actually.