Absolutely NEVER mark anything from an online email provider you want to keep as spam. They use shared systems, it’s not just spam for you, but potentially for everyone on that email provider. That’s one way to protect people from receiving spam, 100 users marked that same newsletter email as spam? Alright, the newsletter will go to the spam folder for the next 20k users.

If you mark legitimate emails as spam for fun you’re fucking up the system (and give the sender a massive headache if suddenly every @gmail.com receiver puts their emails into the spam folder).