Trying to make DNS changes to enable email to be sent using hMailServer.
My mail server is on a separate machine with a separate IP Address.
I have already added MyDomain.com and an email account
I have create a MX Record with the mail server being mail.domain.com an a priority on 20.
1) But the question is how do I now link this MX record for the domain to my mail server/ mail server IP Address?
2) What changes are needed in hMailServer to complete the process and be able to send emails for the domain?
3) In Settings > SMTP > Delivery of email: What should my configuration here look like?
Based on your second comment that you've tracked it down to reverse PTRs, then you need to have your reverse zone have the matching pointers for MX records. If you don't run your own reverse DNS then you'd have to ask the entity who does (the people upstream from you owning the netblock) to add these entries for you.
This goes in the reverse zone file as mentioned, not your forward zone. A quick whois based on your comment of 78.129.226.181 shows it's Rapidswitch owned, so I'm going to guess they do your reverse for you and you only get a few IPs of a hosted server to play with - open a support ticket.