Yeah, I’m pretty sure acting as a public DNS server is outside the scope of what was intended for Neth. I developed a module to let it act as a limited public DNS server for the sole purpose of Let’s Encrypt validation (see userguide:let_s_encrypt_acme-dns [NethServer Wiki]), but I don’t think that’s as general-purpose as you’re intending.
As to mail configuration specifically, this might help:
You’d need to set the SRV record on your authoritative DNS host, but this would automate the rest of the configuration.