Mail relay issue with 10.5.8 Server
I've recently split my network up, with one range of IP addresses for internal, private traffic, connecting out through a fibre connection, and a separate, shared network for general client use, connected via VDSL. They share the physical network, and the mail server sits on the client side. To complicate matters, the fibre connection by default doesn't allow SMTP traffic on port 25, which means if I'm to use the mail server to send (I can obviously receive IMAP mail) then I need to find a way to bridge between networks, which I've done. On the private side, there's a Lion Server set up to relay to the second port (and IP address) of the Leopard server running IMAP and SMTP, logging in to the main account we use.
On the private side all works well - I connect to SMTP on the Lion Server, and woosh, the message is sent to the Leopard server on the other side. I know it has been, because it appears in the Sent folder of the correct account along with everything else that leaves the building. Except it doesn't leave - mail sent from the client network exits fine, mail sent from outside the building finds the SMTP server with correct authentication, but mail relayed from the private network goes nowhere - even though it says it's been sent.
Any thoughts? Why would the SMTP server be treating relays any differently from other mail inside and outside the LAN?
Thanks in advance.
Final Cut Pro X, OS X Yosemite (10.10.5), DaVinci Resolve, Cubix, many Macs