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Features / Considerations
Is it truly a requirement not to reveal users' email addresses to each other?
Authorization: Who can send a message to whom? Is it OK to allow any user to contact any other user, or should there be some kind of acceptance first (as in Facebook)?
Do users want a web-browser based email experience, or should messaging be relegated to users' mail clients?
Event notification: Is this just for messages initiated by users, or should it support notification of events in Synapse (e.g. entity deletion, entity sharing...)
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