I have faced this issue a number of times and you have two options. One, talk to your ISP’s about setting you up with an Autonomous System number for BGP. I have seen this work many times over the years. Just one caution, you will need an interprise level router as the routing tables are large and will over run most routers.
Two, check out a product called Fat Pipe Warp this unit can take up to four ISP connection and load balance between them even if they are of different speeds. You can configure the unit to only have SMTP traffic go in and out of one ISP while all internet trafiic is load balanced across both connection. Here is the web link: http://www.fatpipeinc.com/warp/index.html