I discovered recently that my server has been attempting to send 1000 emails a day (my limit) because of the internal mail system, where it'd be attempting to send actual emails to @ rather then using the internal `mail` that linux has.

Since I don't use this feature at all, how can I completely (either system-wide or per-user) disable this internal `mail` system to prevent it from happening?

Running Debian 8 with EXIM4