In FastMail you could already search based on some of the email headers, like from:thatpurplecompany.com (or has:attachment, https://www.fastmail.fm/help/receive/search.html) and then you could save the search.. It's visually not that rich, but it solves your problem.

Tagging (for me) is more about ad-hoc grouping. As giving it a thought right now it's even hard for me to come up with a realistic use case.