I think, it's not working because working of both WANs should be setted especially (below). So...
0. Hope, you will change lan_ip by yourself

1. Objects > Address book > LocalNetwork
Add objects
lan_mail_server = 10.10.10.15
lan_win_server = 10.10.10.16
lan_servers = 10.10.10.11-10.10.10.19
lan_clients = 10.10.10.100-10.10.10.200 # it will be used for DHCP pool
2. System > DHCP servers
Add new DHCP server, use lan_clients as pool, set lan_ip as default gw, your Win server as DNS (i think, you have AD there?)
3. Interfaces > PPPoE
Add your ISP connections. For example, let it be named wan1_pppoe and wan2_pppoe
For wan1, keep "Add route" checkbox, for wan2, deselect it
[Q] This example is static by interfaces. Do you need a favorier? I mean, when wan1 is down, lan clients can go thru wan2 and same for servers.
4. Objects > Address book > IntrefaceAddresses
Add new IP4 group wans_ips = wan1_pppoe_ip + wan2_pppoe_ip
5. Interface > Interface groups
Add group named wans = wan1_pppoe + wan2_pppoe
It will be used for simular wans rules (ex, external ping)
6. Interfaces > Ethernet
Disable DHCP for wans
[Q] If you don't need to access physical wans, unselect checkboxes "add route" in wan1/2 settings
If you need it, let me know addresses later
7. Routing > Routing tables
Add new routing table alt_wan1_pppoe
Add into one route - network: all-nets, interface: wan1_pppoe, metric: 100
Do the same for wan2 (routing table name alt_wan2, route to wan2)
8. Routing > Routing rules
# process requests from wan1_pppoe
wan1_pppoe/all-nets any/all-nets, forward main, return alt_wan1_pppoe
# process requests from wan2_pppoe
wan2_pppoe/all-nets any/all-nets, forward main, return alt_wan2_pppoe
# change outgoing interface for servers
lan/lan_servers wans/all-nets, forward alt_wan2, return main
9. Objects > Services
Add new group allowed_mail_server = http, https, pop3, smtp, imap
Add new group allowed_win_server = pptp-suite, rdp
10. Rules > IP rules
# allow external ping
Allow wans/all-nets core/wans_ips ping-inbound
# rules for publishing mail server
SAT wan2/all-nets core/wan2_ip allowed_mail_server (SAT: new destination = lan_mail_server)
NAT wan2/all-nets core/wan2_ip allowed_mail_server
# rules for publishing win server
SAT wan2/all-nets core/wan2_ip allowed_win_server (SAT: new destination = lan_win_server)
NAT wan2/all-nets core/wan2_ip allowed_win_server
11. Rules > IP rules > lan_to_wan1
Change destination interface wan1 to wans
Change allow_startard rule - service all_services instead of all_tcpudp
As result, it will work by your requirements, but without additional flexibility (i mean favorier).
Dividing of wan (wan2 - for servers, wan1 - for clients) is performed by PBR and it allow you to have simular IP rules
Please answer my questions and i'll let you know what to change.