Just a heads up. Have you considered a second party system like yahoo groups? We have set up 2 good newsletter systems all ready to go for our 1,000 to 5,000 member lists for 2 years now. But we have never switched the switch from yahoo groups because we are not sure the benefits of a inhouse system outweight yahoo right at this moment.
1. To sign up via the web, new people have to go through the yahoo reg rigmorole if they do not already have a yahoo id. Subscribing by email though is much easier and relatively painless.
2. Sometimes yahoo inserts a 2 line ad, but so far they have not been noticeable on our lists.
Limited upkeep: no regular admin tasks like bounce control, spam control, space or memory or smtp restictions on your own server, trying to work out small bugs, security measures, sometimes we get people subscribing direct from yahoo directory of lists, etc.
As i have said we have been ready to go and have tested two very good systems - one php and one perl, but so far yahoo groups still is so good that it does not justify the extra work and admin time. We are keeping the others ready to go for the time when Y! introduces payment for groups (even a nominal payment we would consider) or suffocates the thing with advertising.
To answer your question direct, I second the recomendation abve for Mojo Mail, from our trials, though we have never tried it in a working environment with all 7,000 members.