Please critique this email database schema
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Could you please critique this first attempt at a schema for an email database?  As you might expect, an email can be sent to one or more recipients via the To:, CC:, or BCC: lines. What I'm unclear on is providing an ability to show the thread between two users on a specific topic, hence the 'in_reply_to_msg_id' column in the Message table.  I'm new to this and appreciate your comments and suggestions.

<a hef="http://imgur.com/JQZ6x">email database schema</a>
