You can use a Sonicwall, for instance, to do a "round-robin" load-balance thing, but you might need to dedicate the Sonicwall to SM if you don't want the rest of your LAN to be load-balanced that way also. And then (because all accounts would be perhaps sending from all gateways), as Webio appears to point out, you could run the risk of ALL your gateways being blacklisted if an account is compromised. For strictly load-balancing, you can find an inexpensive, used older Sonicwall on ebay, for instance, as long as it has some version of their Enhanced firmware AND can handle the throughput you need.
Lastly, if you are using multiple ISPs for gateway IPs, remember that your SM server needs to access nameservers that will work with all your ISPs, such as Google DNS or OpenDNS. Most ISPs these days limit use of their nameservers to only their circuits.