When you say "bond", I think you mean what's actually called "load balance" - i.e. distribute the load between your two ISPs so that in effect, your devices use their combined speed... That's what you want, right? Or is this "bonding" thing something else?
To force the SMTP traffic to the first ISP, you can use
per-traffic load balancing, and you can use
PCC to effectively combine your ISPs' speeds, by telling the router how many connections to be passed to which ISP, so for example, if both ISPs offer the same speed, you'll want to give half of all connections to the first, and half for the second ISP.
Keep in mind that with this approach, each connection could theoretically deliver only the selected ISPs top speed (e.g. if each ISP offers 100M/100M, you have 200M/200M for your devices, but even if there's only one device online making one connection, that one connection will only have a speed of 100M/100M, even if the device is allowed more; the device can still consume more if it makes another connection, which would go to the second ISP).
EDIT: OK...
this manual page makes me realize "bonding" means exactly a variant of load balancing that doesn't have the above drawback. I don't think what you want is possible though, as each ISP will more than likely not forward packets for a connection that wasn't initiated through them, plus the remote host requires packets to have the same IP:port combo to count them as the same connection, and your two ISPs have different IPs.
