I want to loadbalance outbound emails via multiple IPs, is there any built-in tool or some easy way to do it with postfix, sendmail or exim? for example if I assign three IPs to the email server and emails are sent via all the ips one by one.
If you want send mails from different Ip's from a single mail server than you should think about postfix multiple instance on a single host. And you can assign a different Ip address for each instance and your null instance will submit mails for each instance. for more detail you may refer these links This one for how to assign Ip and all http://souptonuts.sourceforge.net/postfix_sbr.html and this for all basic information how we create and how instance works http://www.postfix.org/MULTI_INSTANCE_README.html