A customer wanted a system to send a large number of emails to their users. Whenever he liked.
He wanted this system to be smart. It means sending the emails to the user individually and it looks like the email was sent to that person. For example, names, specifications, descriptions, and anything in the email could be dynamic and could be defined by the admin. That is, for each user, the text of the email was different and worked automatically.
Once, I used this system for one of my sites and sent an email to a large number of users. My site rank from 29532 reached to 2999. It was a very good rank. The reason people came to the site was that they thought this was just their own email. Because the sending method was such that every user who saw the email thought the same.