Can you help me find email list software like this?

1.I am looking to set up a service that will email a monthly newsletter for multiple companies is a specific industry (s).
2.I want each company to be able to log-in and maintain their own list.

For example: I will be offering a generic monthly email newsletter about ‘Jewelry’. I will offer this newsletter service to multiple companies who sell jewelry so they will have an easy way to keep in touch with their customers. I will only customize the heading for them. I want them to be able to log-on and create & maintain their own mailing list.

Any idea on software that will allow my customers to log-on and maintain their own email lists?

Thanks!