This seems very difficult. Any tips to decreasethe time to dothis? The problem seems to be in the population of the addresseses in mail.app.

If you are sending 600+ emails, then Mail itself isn't a good idea - nevermind combining Daylite + Mail (lots of unnecessary overhead).

A smarter, more efficient solution is to use Direct Mail from e3 Software along with the Direct Mail plugin (on our addons page).

Find out more here.

They just updated their Daylite plugin to version 2.1.