Ive made one of these with Variable v4. It was more of a PM system, but consisted of the same things. Ill explain it for the sake of it.
Differences:
Uses v4 Variables
+ Hides your chat message
+ Can be done anywhere in the server
+ PMs can be read anywhere in the server
+ Pms are instant
- No saving
- Alot harder to add new 'contacts'
For hiding the chat message, i used "StoreNextMsg" event. Its a old one, cant be found. I could PM you it if you want. It just makes the next chat message you will use be set to a v4 variable of your choice, but i mainly used it for muting other players >:D
Ontopic: This is a basic idea, put into a neat format. Well done. Maybe add reading emails from anywhere?