I have had the same problem, At one point my Multi-tenant server was sending out well over 1000 of these emails at least once a week. 3CX recommended i clear the Application Log in event viewer and after i did that it stopped until the application log was full once again.
Since then i have set the log to "Archive the log when full, do not overwrite events" instead of "Overwrite events as needed (oldest events first)" since then i have not gotten any of these old emails.