Rich Felker on Nostr: My super spicy take: labeling a sender's mail spam when it's not is libel (as a mail ...

My super spicy take: labeling a sender's mail spam when it's not is libel (as a mail provider presenting that label to your users).

If you find it sus, you have a right to reject that at smtp transaction time so sender knows and can correct any issues or use another channel to reach the recipient. You don't have a right to call them a spammer without being able to back that up with evidence.