Email List Encryption - Problem

Scenario:

1,000 email discussion list users who are trading information regarding
private health issues in women and children who have been victimized by
domestic violence. The information does not contain SSN or ID by name
(HIPAA compliant), each user wants to use their own email address and
client of choice. Medical, law enforcement, volunteers, emergency room
personnel, etc are typical users.

The problem is how to encrypt the data from start point to end point
(Sender to Recipient):

a) *without* installing encryption at the Sender's machine.
b) with installing encryption at the Sender/Recipient's machine yet the
program encrypts and decrypts automatically with no user interaction and
invisibly (to the typical user).

The users have resisted PGP, closed system emailing and any other type of
active participation, in short, they want security without any appreciable
effort.

Comments appreciated. Advise going to a non profit.

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