Slightly complicated set-up, will do my best to explain. All fairly cobbled together.

I am running my own mailserver (using hMailserver), has been up and running for a couple of years, working fine. I have a Dyndns.org URL, which keeps the IP address accurate with the MX records etc, no problem with any of this.

I just received an updated modem from Orcon, as I am upgrading from ADSL2+ to VDSL. Set up all the port forwarding to replicate the old modem, and put it in place.

All looks like it's working fine, can RDP in from externally, so the port forwarding is definitely working.

However, INTERNALLY on the LAN, I suddenly can't check the email over IMAP. It announces that it can't connect to the server.

The email clients had been set to my dynamic URL, and had always resolved fine, even when locally. It always used to just identify my router as having the appropriate external IP address, and then port forward from there to the server. However, it seems like with this new router, this isn't working. I've run through all the configuration options I can see, turned off all firewalls etc, but still no luck. Router is a NF4V provided by Orcon, running on their Genius platform.

I had an email client on the same machine as the mail server, which was also connecting to the dynamic URL and failing. When I set it to 127.0.0.1, it works fine. However, pinging the dynamic URL also works fine, as does the RDP connection, so the issue isn't with the DNS resolution.

Any thoughts of how to make this work? Setting to the local IP address won't work, because I need to take several of these devices outside the LAN.