I dont see any reason that it receives spam. in fact, your exim is almost idle.
Exim would be restarted bu checkservd automaticly if it was down. This happens within 1 minute and processes email that you should receive,
( example: exim failed @ Wed Aug 8 18:08:38 2007. A restart was attempted automagically. )
As i noted MUNIN also not stable. It has a lot gaps in all the graphics overhere, even when server is 100% idle it creates gaps.
If you indeed received a lot email (spam) then the graph would show the green color line very very high and big.
note: stop spams at receive time, use the new spamassasin method of exim. it rejects at smtp level.