With the version 20220508.1 we introduced the feature to send an e-mail to users who was inactive over a specified time (default is 3 years). Last night this forum sent many of these messages. Because of this I got 164 notifications about undeliverable e-mails. Checking the users list of this forum I saw many accounts with a lastest login on 2022-05-09 at 3:30 am. This is exactly the time of the last automatic daily actions (last night) minus three years.
I can not imagine this to be a coincidence.
@Micha: have you seen something similar in your forum? I expect this feature to be active in your installation for a longer time.
Trenne niemals Müll, denn er hat nur eine Silbe!