Subnet getting blocked

[SurrealTiggi]

Hi guys, hope someone can assist with a small problem I'm having.

I've got a Zimbra server behind a gateway, local address 192.168.100.30, gw 192.168.100.240.
I've other machines running through a different gateway (192.168.100.254), but still on the same 100.0 network.

Due to a lack of IPs, I've created a 105.0/24 address on the .254 gateway. All the routing has been setup as well, everything is accessible to and from 100.0 and 105.0
Except email. If I do a port scan from a 105.0 address I can see all open ports on the mail server, but still can't access it at all. SSH, POP3, HTTPS, SMTP, and so on, seem to be completely blocked.

Is there something I've missed?

Any help is appreciated.