Thanks for your help, Mr Maury. By the complexity of your solution I am not so ashamed of asking a silly question yesterday. I am not familiar with database maintenance, so that might be why your solution seems complex to me.
First of all though, I encountered a warning from phpBB3 when I disabled boardwide emails:
Information
Configuration updated successfully.
Please note that the currently selected activation method requires emails to be enabled, otherwise registration will be disabled. We recommend to either select a different activation method or reenable emails.
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I presume that would be eliminated after the SELECT operation, although I don't quite understand how the automatic email authentification for new users might work if the list of user email addresses is a static, manually derived list as I suppose you've suggested.
On the understanding that the SELECT part of the solution is an SQL instructon that is performed outside the ACP, I guess on the command line in MYSQL?, or using phpmyadmin, then would it be fair to conclude that there is no known way to disable user-to-user email via the ACP. It can only be done by adjusting the database from outside phpBB3.
I guess we can just permanently suspend anyone who tries to send unsolicited emails to other users. That ought to work, retrospectively anyway. It just seems a little strange and I wonder if my memory is failing me in my old age. I was sure that either I've just always disabled interuser emails or else the default config' was set that way from the beginning. I don't remember any of the phpBB forums I've run in the past decade having emails between users activated. Even if it's through the board, and the email address itself is kept private from the sender, (Users send email via board), there is still a means for one user to in some ways troll another, and that is what I really think is a high cost for a minimal benefit when there is a perfectly functional PM facility available inside the forum.
I don't think I want to start playing around with databases when I really don't know what I am doing in MySQL. I just setup the database with phpmyadmin and let phpBB3 run everything from there. The database is a bit like a black box to me. I just don't have even the basic coding skills in MySQL, so if I can't disable those emails inside the ACP, then I'm going to have to live with them.
Sorry to ask a question that has no answer suitable for a MySQL ignoramus such as I. I hoped it was somewhere in the ACP.