It really depends on what you want to do. For example:
When do you want to send messages to the user (on login or when the message is created)?
Would you rather use the Manager message system or email to send the message?
Do you need to store the user message, or just send it?
Will you need to store multiple messages for each user?
How do you want the user to respond to the message?
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PLEASE, PLEASE specify the version of MODX you are using . . . PLEASE!
MODX info for everyone:
http://bobsguides.com/modx.html