I use Green Arrow engine + studio, by the folks at drh.net, and it can do that very well. You can basically get a hook that will execute your arbitrary code fo

I use Green Arrow engine + studio, by the folks at drh.net, and it can do that very well. You can basically get a hook that will execute your arbitrary code for each contact - database lookup on a remote service, whatever. So you can build very complex newsletters that are totally customized to each recipient. And it is fast as can be (can pretty easily push 1M+ messages per hour if you have that much volume to send) In order to take full advantage of it you need to be a developer though. It's got great pricing too, esp. if you get their perpetual license and then run it on your own dedicated server (they set it up.)