1) First, you need to process connections from each WAN separately.
Routing > Routing tables
Create new one named wan2
Create route (interface, network, gateway, metric)
wan2 all-nets wan2_gw 100
Routing > Routing rules
wan2/all-nets any/all-nets, forward main, return wan2
I recommend you to do the same for wan1, but it's not mandatoroy
2) As i understand, your DMZ is "gray", right?
3) Rules > wan1_to_dmz, wan2_to_dmz
SAT/Allow rules should be with wan1/all-nets core/wan1_ip networks (for wan2 - replace wan interface)
4) Rules > dmz_to_wan1, dmz_to_wan2
Allow_SMTP rules not working because your traffic passed thru NAT all_tcpudp, so you can remove this rules
5) Rules > Access
I don't see this items are necessary
6) Routing > Routing tables > main
Add routes (interface, network, metric)
core Wan1_VPNServer 0
core Wan1_WebServer 0
core Wan2_VPNServer 0
core Wan2_WebServer 0