Well, that cost is almost certainly lower than building a solution to figure out who is reading stuff or anything like that. Sending emails just isn't expensive

Well, that cost is almost certainly lower than building a solution to figure out who is reading stuff or anything like that. Sending emails just isn't expensive resource-wise. As for spam detectors: Just forget about spam detectors. If people employ someone to throw away their emails for idiotic reasons, that is not my problem. If they want to be subscribed to my mailing list, they should not employ people who throw away my emails.