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SRP521W port forward through VPN


I have two SRP521W.

One in HeadQuarters (HQ) and one in Branch Office (BO). Both have Static Public IP

Site-to-Site VPN connection is estabilished between them. VPN works.

In HQ we have mail server. When main internet connection fails backup connection starts on 3G modem. VPN on 3G modem is working. But we have problems with emails. If main internet connection fails in HQ we dont get any emails. 90% of sent emails are delivered through 3G - 10% are blocked by

recipient email servers (blocked as spam - IP not equal to our MX).

So I figured out to forward emails through our second static IP in Branch Office and add this IP  to DNS as MX with lower priority.

Question is

1. How to configure Branch Office SRP521 to forward port 25 to server in HQ using VPN tunnel.

2. How to configure HQ SRP521 to forward port 25 to router in our BO. All remaining traffic (http) should go out through 3G modem).

Branch Office local router IP is

Assume Branch Office Public IP is

HQ local router IP is

email server local IP is

Assume HQ Public IP is

in BO (branch office) I created NAT forward port 25 to - but it doesnt work.

What else should I configure.

Greetings, Szymon Polok

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