then you need:
- your MX record to point to your public (static) IP (you’d use a dyndns host too, if is it the case)
- your 25 and 587 ports exposed to wan
- your public domain (the real one) created into NS
- your real email addresses created into NS
- your email clients to use NS as email server (imap and smtp) configured for your public email addresses…
with this configuration, internal email will work as expected (not routed externally) and public email too…
If you use some public/big ISP like TIM or Fa®tweb here in italy, I strongly suggest you to buy a smtp service and use a good smarthost to send your emails to the world, otherwise you’ll be in trouble with BL almost every day.
my 2c