poMMo is a versatile Newsletter and Mass Mailing software solution that can be installed, configured and run on any php server within a few hours (excluding skinning). You can reskin the entire system within about 48 hours if you know your code.
Some of the things that makes poMMo so cool is the lack of bloatware needed to handle newsletters and mass mailing, the incredibly tiny footprint, the quick loading times, and the simple but very functional layout. Within a few hours you will have grasped how to configure your system, load your email lists and create your first newsletters.
poMMo can handle very large email lists upward of 20k addresses and can be installed alongside your existing website in a matter of minutes. Just make sure you create a new database for it and away you go.