So I set up push notifications from my Raspberry Pi to my Android phone, to know when exim sent a mail locally.
The easy part was the actual push notifications.
The more tedious part turned out to be looking for a way to send a notification for each mail.
After some research, Procmail seemed to be the only fitting tool to pass info to a command (in order to give the push notification some content so I know what's up)

In the middle of everything, I managed to fuck up exim system-wide, so mails didn't work, which was fucking great of course...

The magic receipt is this:
:0 c
| ${NOTIFY} -t "Pi mail: $SUBJECT"

Anyway, this is the result (using a test mail by mdadm + an actual degraded array I am still waiting on replacement drives for):