@DJBenson:
Am I right in assuming if I use a port alias containing 25,465 and 587 and assign that to the rule you suggested, any other traffic originating from the mail server will still load balance (i.e. normal HTTP/S traffic)?
Yes, set the destination for smtp, etc and the other traffic will progress to the default rule. You may want to put https on a failover group (not load balanced) to avoid the problems you saw with banking sites, etc.