One-Way Listserve needed

I'm in need of a FREE listserv service where we can send out messages to about 500 of our organization's members, but that it's only one-way. We'd like to be able to have members unsubscribe, but we'd control and update who subscribes. It's essential that we'd be able to upload membership emails via a spreadsheet or other means.

Right now, our listserv is 2-way, which has been clogging up the inboxes of the membership, so we've set up a social network (and invited in the world) to handle the good ideas and sharing of the membership.

Any help you could offer would be appreciated!