Yes, they do monitor what's in your mail when someone reports you. I've been banned for receiving golds from one of my friend coz he bought gold from guy on the server, and those gold were gather by hacking into others account. Once those account gets reported blizz will try to track how these gold were spent and sent.
If it's farming mats I think it'd work the same way, so as long as your mats are CLEAN (not from any hacked account) I believe blizz will not bother with monitoring your mailbox 24/7. Just what I think so don't quote me on that.