Force SMTP Traffic to go out over VPN to alter Gateway

[sc0ttjm]

Hi,

My customer has 2 mail services on 1 server, I have migrated them from an on-premises Hyper-V infrastructure to a new UK Data Centre Hyper-V infrastructure so they now have a different External IP address.

I have changed the IP address associated with the MX record for 1 service but cannot change the other.

I have an IPSEC VPN between the 2 sites and I can get email to service 2 as it is forwarded using NAT on the Customer Site router, but we cannot send as the mail goes out using the new address.

Is there any way on the OPNsense router in the DC, to force SMTP traffic out for email service 2 over the VPN so goes out using the old IP address?

If not, can I force ALL SMTP traffic out over the VPN so they both use the old IP address?

Network Diagram attached.