I am looking for an email software that can do throttling, be able to control how many connections to each of the receiving servers. Have about 100 000 emails to send each week for an MNC, everytime I run the apps, basically all the ISPs start tarpitting and grey listing. Even Yahoo, Hotmail and Gmail. Need a solution whereby the emails can be send gradually and avoid such traps.

Consider PHPList, which does support throttling. http://www.phplist.com/

The interface is somewhat hard to get used to ... but it does its job real well.

Postfix can do it.

Older versions of Mdaemon can do it, by controlling number of connections and number of messages per connection. Status of newer versions is unknown.

You may find that the native smtp service has knobs you can tweak for this purpose.

Are you looking for a managed solution ? or a how to ?
There's a few ways to do it. Reputation also should be monitored for your ips.