(a) never use the subject line, and just send you a succession of emails with blank ones

(b) when they have a new issue/problem/task they want you to deal with, instead of opening a new email and leaving the subject line blank, just reply to the last email they had from you, so you have an email with a subject line entirely unrelated to it

(c) set read-receipts on everything they send you

(d) THIS MESSAGE IS MARKED 'HIGH IMPORTANCE'