HI There
this looks like a DNS related issues I had not so long ago,
theses are the steps I follow in resolving my issues
firstly request from your ISP/ or where ever you getting your public ip address from that they setup reverse dns lookups on there side
then you need to setup A records on your dns servers to point to that ip address
once that's sorted test to make sure your ip address resolves to the correct FQDN
and that you FQDN resolves to your ip
lastly setup a mx record that will point to your email server (iredMail)