Over time, users invariably change email addresses and usually do not bother to update their phpBB account.
However, users will usually have selected email as the preferred method of notification, so they will receive emails whenever they receive a private message etc. Additionally, the same users may have subscribed to multiple fora or topics or requested notification to replies to their posts. This can lead to a very large number of email notifications being sent out to non-existent email addresses and then an equally large number of emails in response indicating that the message could not be delivered.
Typically, the administrator of a mailing list is considered responsible for ensuring that email details are up to date and to remove any stale email addresses from the list to avoid unnecessary load on the recipient mail server. If no steps are taken to clean the mailing list and messages continue to be sent to non-existent addresses, your email address or mail server IP (for most people this is the same as their phpBB server IP) could be added to a blocklist or your IP reputation score could be downgraded to the point where your notification emails to genuine emails are marked as spam/junk.
I have examples of users who have subscribed to new topics in every forum, so for every new post there an email going out to a non-existent address. This can add up to a massive amount of outgoing junk email.
With the standard features of phpBB, it is not possible to stop emails being sent to a user even if you are aware their email address has gone stale. There is no way for an admin to change a user's notification preferences through the ACP, so you would need to manipulate the database through the backend for any user identified as having a stale address.
One possibility is to PM each affected user asking them to update their address. However, this is time consuming and is rarely fruitful in my experience. Whenever I have done this, most users ignore it. Often, the offending users are the ones who haven't visited in years - they won't be taking any action to update their contact details but it would be unfair to ban them simply because they changed email addresses some time ago.
An extension which:
- adds an option within the ACP (perhaps in the User Management screen for each affected user) to mark a user email address as stale.
- adds a notification for the user to update their email address with a link to the relevant screen in the UCP. The user would then see the notification the next time they log in.
- stops notifications being sent by email until action is taken to change the user email address. To do, I suggest intervening in the email process rather than changing the user's notification settings (so that the preferences remain the same after the user has provided updated contact details).
- after successful activation following email change, the email ban is lifted and everything goes back to normal.
I considered this as a possible improvement to phpBB core code, as it is pretty common for online platforms to roll out reminders to users to update contact details. But perhaps it's best to try and get it working as an extension first. I'll leave this for others to provide their input.