Required

Embeddable Signup box that can be embedded in any site via HTML - so that capture of emails is very easy
Function is broadcasting announcements
Allows users to unsubscribe themselves, via a link at the end of an email.
Infinitely scalable, and immediately usable with 100,000 users
Well documented and well-supported software, with good potential of long life (decades into the future)
An OSE-specific instructional is available for managing the list
Allows unlimited sending of messages
Once a person signs up themselves, email spam filters do not catch emails
Allows a backup dump of all subscribers
Allows plain text
One way list
Bounced message reports are available via interface

Optional

Easy interface for managing message composition, preferably with a standard template for emails
Allows HTML embed in email
@opensourceecology.org domain name - if possible

Removes users automatically if mail address fails permanently (no longer a valid email)