Re: MX Records newby
MX records are never an IP address. They are a host. The host can be anything that you like your own domain. mail.example.com is a common choice. What the host name is doesn’t matter, as long as it resolves. I have used owa.example.com in the past as that was being used for OWA access and resolved correctly.
You shouldn’t need to enable anything in Exchange for SMTP delivery, it should be already set. You just need to open port 25 on your router.
Reverse DNS/PTR records are for outbound email only, nothing to do with inbound email. If you send email via your ISPs SMTP server as a smart host then you don’t need to worry about it.
I am not a fan of fallback settings at the ISP as I find they cause more problems than they fix. They are also a spam trap and you have to have ways of getting the email from the ISP to your server, which usually means the POP3 connector and its drawbacks.