Hi ,
1 .NAT mail server or Exchange server local IP with ISP B Public IP
2. NAT Your LAN Proxy server / ISA server Local IP with ISAP A Public IP
3 . Do a route -map on the Gwateway router to force ISP A IP's go through ISP A and ISP B IP's o go through ISP B .
Another way will be doing one ACL to match http traffic and another ACL to match smtp traffic and do a PAT (NAT overload ) making this accesss-list as souce list with ISP A and B
I hope this helps
Regards,
Haris