Under LAN of Site A. I tried setting rule: SRC * DST * DSTPort 25 GW OPTVPN
That looks reasonable.
and also SRC Port 25 DST * DSTPort * GW OPTVPN
Setting a source port is almost never right, and is certainly not right in this case.
I have no problem routing inbound internet traffic -> 99.99.99.99:STMP to 10.10.0.15
So if that is the case, you want to check:
The rules on the OpenVPN tab/interface at Site B to be sure the traffic is allowed from site A (10.10.0.15) to any
You have outbound NAT in place on WAN at site B for the 10.10.0.15 source address. That is also where you would specify 99.99.99.99 as the source address if there is more than one choice.