My experience is that you send email to someone whose Op Sec is not as good as your own, then your email will be harvested at the point when that person's addre

My experience is that you send email to someone whose Op Sec is not as good as your own, then your email will be harvested at the point when that person's address book is harvested. I don't know all the details of how these harvests occur, but using a shady mobile app with Contacts permission would be enough.