Balancing outgoing SMTP relay

[freebsd]

Hi
 I have a simple question but googling does not lead to a valid/usable
answer.
I need to load balance OUTGOING emails. I have serveral smart hosts. I need
my internal SMTP server to send mail using ALL of the smart hosts together,
making some kind of load balancing (no need for wheighted one).
Someone pointed out to use a name for the smart host, and have DNS to
resolve that name to the IP of all the relays (multiple A records) but this
turned out in doing failover, not load balancing.
Anyone has a *working* idea for solving this apparently simple problem?
Thanks