Hello
We have 2 sites (Site A and Site B) and both have Draytek Vigor 2830 routers, we are using a static IP at Site A and there is a Dynamic IP at Site B
We have a LAN-LAN VPN using IPsec between these sites, initiated by Site B

Configuration:
Site A (Static IP for WAN)
LAN: 192.168.100.0/24

Site B (Dynamic IP for WAN)
LAN: 192.168.110.0/24

Here is the problem
We have a mail server at Site B which cannot be moved and need to route SMTP traffic through Site A and over the VPN to an IP (let's say 192.168.110.1) in Site B. so far I have tried to create a NAT rule at Site A which sends traffic for Port 25 to the IP address in Site B but this doesn't seem to work. When we test to try and connect to port 25 this fails.

So my question is, how can I successfully route traffic (SMTP) over an IPsec VPN?

Thanks