Phostal is a mailing list manager. A couple of companies I've worked for use it to send newsletters and promotional messages out to around 500,000 subscribers at least once a month.
I need to check the legal status before I can post the code here. There's also the possibility of developing Phostal into a commercial service so it may never actually appear here. If you're interested in knowing more or would like to talk about possible licensing or ASP usage please feel free to contact me.