>>we now want to use a professional newsletter management system
PHPlist is worth having a look. It has all the features a professional newsletter management system should have, but is not very fast (5.000 emails per hour on one server, with good scalability in web server cluster)
I use a hosted Lyris service for sending out monthly flyers/newsletters. I think it's quite reasonable at well under $100 a month. There are big differences in functionality between systems. One of my main criteria is bounce management. If you are mailing more than a few thousand names, manually dealing with bounces is a time-consuming chore. Easy and fully automated subscription/unsubscription is important, too.
PhpList is an excellent suggestion, but you may find it limiting if you wish to start gathering and tracking more information about your subscribers/members. I went open source last year and now use CiviMail, which is a component of CiviCRM. It's not easy to get started with but has proved worth the effort.
Hosted solutions include services like GetActive, if you're in NPO-style activism, and other well known providers if you're not.