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Even changing config ... forums have already sent the mail ... Elvis left the building.
I had this happen about a week or so ago with a newbie Moodle admin ... thankful that I had Postfix in front of any other smarthost (smtp relay) cause I let Moodle do it's thing and manually cleared the mailques as messages came in thus preventing the moodle server from getting blacklisted god knows where. And didn't mess with config of forums .... 125 of them ... nor the related tables. Did have a long talk with the newbie Moodle admin though. Not his fault ... he didn't know that 8000+ users were subscribed to a hidden forum that had never been used by former Moodle Admin.
Not ideal ... a pain in the arse ... but ....
'spirit of sharing', Ken