I just noticed that the system accounts system and discobot have no mail address (system has no_email and discobot has discobot_email).
When I tried to enter a mail address for system Discourse says that a mail with a confirmation link is sent. Which is not, I see an error message in the log:
confirm_old_email
550 5.1.1 <no_email>: Recipient address rejected: User unknown in virtual mailbox table
Why do I ask: I noticed that system got some messages from users and I did not notice it. Is it a problem on my side because I did not see notifications or are there no notifications if this/these users receive communication from users?