I have a Vigor 2830 series with 8 IP's (x.x.x.40-47)

The routers IP is x.x.x.41

WAN2 (x.x.x.42) has several ports NAT'ed to a mail server, 192.168.10.11

I also have a server sat in a DMZ on WAN3 (x.x.x.43)

My issue is that although MX, SPF and reverse DNS all point to WAN2 for the mail server, outbound mail appears to come from the routers IP address (x.x.x.41) which is causing issues with spam filters.

How do I get outbound email to route through WAN2?

Have you tried using Address Mapping?

NAT -> Address Mapping

Fix the external, WAN Interface and Internal IP to the desired settings, then try it.