Answer
Check your custom Events first — by default, the large majority of emails sent out of IFS originate from Events (specifically event actions of action type Mail), rather than any other single mechanism, so an unexpected new email is very often traceable to an event that's now firing (or firing differently) since the Mail Sender was configured and able to actually deliver mail for the first time. Look at the relevant application message's own details — specifically the Sender field near the top of the message — which typically identifies which event or mechanism actually generated that specific message, giving you a concrete starting point to trace back to the responsible event and its mail action.