Some years ago I run into a e-mail loop. An e-mail request is sent to a marketing e-mail service. For a specific e-mail notification, we do not receive a confi

Some years ago I run into a e-mail loop. An e-mail request is sent to a marketing e-mail service. For a specific e-mail notification, we do not receive a confirmation from the marketing e-mail service that the e-mail reached the recipient. So our service re-sends the e-mail. This goes on for several days. It turned out a filter did not let the notification reach our end. By the time we figured it out, someone received thousands of the same marketing e-mail.