For multihoming you should have your own public address space. If you do not own the address space it is more likely that ISP 1 would drop the packets for ISP2 subnets and vice versa
If you own the address space, you can force the mail traffic on the outbound direction but we cannot guarantee the inbound direction of the traffic
eg config
interface Ethernet0/1
Description inside
ip address 10.10.10.1 255.255.255.0
ip policy route-map mail
interface serial 0/1
description " ISP1 link "
ip address 20.20.20.1 255.255.255.252
interface serial 0/2
description " ISP1 link "
ip address 30.30.30.1 255.255.255.252
ip route 0.0.0.0 0.0.0.0 30.30.30.2
ip route 0.0.0.0 0.0.0.0 20.20.20.1 20
route-map mail permit 10
match ip address 1
set ip next-hop 20.20.20.2
access-list 1 permit host
HTH
Narayan