Eudora & eMailer (and other email clients) use a protocol called "PASS" to change a user's password.
AppleShare IP 6.x comes with "PASS off by default. This is because "PASS" is a "clear-text" based protocol with limited security support. Because of this, it was decided to make the administrator explicitly enable this protocol.
To enable "PASS", go to the "Server" menu in the Mail Admin, choose "Advanced Settings" (The Mail Server must be running to do this.) and click on the "Protocols" tab. Make sure that the "PASS" protocol is enabled.