If I configure Gmail to retrieve mail from a different provider via POP3, can the retrieved messages be identified in a unique way in a filter to be tagged with a specific label e.g. "pop3"?
I am aware of deliveredto:
but I cannot use it in this case because it is not specific enough for a catchall custom domain that also sometimes forwards to the same Gmail account (yes, it is an unusual setup but let's not discuss that part in this question).