I've been doing some research into this, but my experience as yet is with the smaller scale subscriber base, say sub 4,000.
What kind of software do you need when the subscriber base gets to a larger size, like 100,000? 1,000,000? I've got a client who needs this kind of capability, if things go right, and I'd rather not hope for the scenario where they only get a very small percentage of their potential subscriber base.
Majordomo seems like something that might work...I've also read it's nothing short of a large headache. Preferably, web based adminstration, and a MySQL backed database integration...if I'm really wishing, the software would be in Perl.