Murray,

Odds are that if you send out enough spam to enough people, your spam message about a UPS delivery (for example) will correspond to a household that is actually EXPECTING a UPS delivery, so will unwittingly open it. Same with bank names, regular online sales places, etc. Eventually someone who actually is waiting on an Amazon transaction will receive a spam timed serendipitously to correspond with their willingness to open the message and read it or respond.

SRS