I am not sure this works anymore or if it ever did since I heard about it years ago, but when you send an email to someone on something like gmail who doesn’t use some form of adblocker, be sure to put some hidden text in whatever color the background of their client is. Apparently they’ll eventually start seeing ads for that specific thing you hid in the email since those emails are most certainly scanned so you can get targeted ads (especially gmail and most likely outlook as well).