After years of using POP from my ISP, I now need to route email from my external site, through a dynamic alias, to my internal network. I do not know how to do this, and I would really appreciate some detailed help.

For years, my external ISP has maintained a publicly accessible domain, and a host that runs a defined set of services. The important service for this discussion is POP. Let’s call this mydomain.com. mydomain.com is actually just an alias for my ISP’s server, called ortcloud.fireserver.net. My ISP will let me create MX records for my domain, and define email forwarders. Other than that, I have limited control of the software running directly on mydomain.com.

I have complete control of the servers on my internal network. I have postfix installed, but idle on a server we can call fedora20.internalnet.org. This server also runs BIND 9.9.4. My internal network has a dynamic external IP address assigned by my evil, internal ISP. For this discussion, let’s say the dynamic address is c-60-190-42-156.hsd7.ca.greedyjerks.net. I currently use dyn.com to redirect some external traffic to my internal network, so I have used dyn.com to create the alias cutealias.dyndns.net. I have put in a ticket for my ISP of mydomain.com to tell me if there is a practical way to configure mydomain.com to redirect to my dynamic address. If that cannot be done, then I will need to configure mydomain.com to redirect to the stable cutealias.dyndns.net, which is the external alias for fedora20.internalnet.org.

In summary, here are the names involved:
External domain and host = mydomain.com = ortcloud.fireserver.net
Dynamic name of private network = c-60-190-42-156.hsd7.ca.greedyjerks.net
Dynamic alias = cutealias.dyndns.net
Internal mail server = fedora20.internalnet.org

So how do I proceed? I looked up “MX record” on Wikipedia, and the web control panel for mydomain.com looks pretty simple. So do I create an MX record with a destination of cutealias.dyndns.net, and open port 25 on my internal firewall? What else must I do fedora20.internalnet.org? What about outbound mail? Also, mydomain.com, which currently gets all my mail, is configured to run the SPF (Sender Policy Framework) system. Do I leave that as is? Do I need to set up SPF on fedora20.internalnet.org as well?

Thanks again for any help.

The standard mandates that the MX record point to an address (A or AAAA) record, not a CNAME alias. That's not a problem, since the MX record can point to any name, inside or outside the domain. You can point it directly at cutealias.dyndns.net.

SPF is all about creating additional records validating the server used for outgoing mail, and if you use the ISP's Smart Host as a relay, no changes are needed.