In response to: It would make sense to check if people are coming back to the conversation or not. Otherwise people will quickly mark it as spam, which if it occurs more often it easily leads to the website getting blacklisted as a spammer. After which emails stop arriving. With the awesome addition of bounce functionality in XF its a shame if websites get blacklisted none the less. So my suggestion is to limit the number of emails that users get by checking if the emails result in users coming back to the content / site. If emails are not pulling users back to the site then they should not receive a mass of emails.