Logic of controllers and how to seperate them

[eluser]UnforgiveNX[/eluser]
Hi all,

I'm a bit confused about the controller structure. I'll give an example to my issue.

Say that I'm building a community site. A user comes and logs in. Users can send messages to each other. Now;

I have a auth controller which has "login, logout, register, forget_pass etc." functions.

When it comes to messaging, should I have a seperate controller as "message" which has "send_message, list_messages etc." methods, or should I use the previous "auth" controller to manage the messages? I can do this in both ways as obvious.

How should I decide about creating a controller?

Thanks