Private Message Restrict limits the set of allowed recipients based on a set of
rules when a user composes a message. A rule may be applied based on a user's
username, role, or organic group and may restrict the user to sending to only
other users with specified usernames, having particular roles, or belonging to
particular organic groups.  For example a rule could be made which would
restrict users with role 'client' to only being able to send to users with role
'firm member'.  If multiple restriction rules apply to a sender then the last
rule applied will take precedence.

One possible application for this module could be in converting private message
into more of a filesharing system.