Ever go looking for an email only to find that it was delivered to your Spam folder? Have you marked an email as Spam but find it landing back in the Inbox? Both of these situations can be frustrating, but there's reasons why email is filtered how it is. Keep in mind the following points:
Yahoo's Spam filters take precedence over any filters or blocked addresses set up on the account. This means that when an email is deemed as Spam, it will go straight there.
When you mark an email as Spam (or not Spam), Yahoo will deliver emails from that email address based on what you clicked. Marking mail as Spam or Not Spam is a strong signal to our system when determining whether other emails should be sent to Spam or not. It's best to mark emails from the web version of Yahoo or using the Yahoo Mail app to make sure the system gets the request.
If you reply to a message, the sender's address will be placed in the Address Book, making it more likely that their emails will be delivered to the Inbox in the future.
For legitimate emails you just don't want to receive anymore, it's best to use the "Unsubscribe" link in the message rather than marking it as Spam. This helps our system determine what's junk and what isn't better.
Keep in mind that just like people and companies, the IP address that an email originate from has a reputation. If emails from an IP address being deemed as Spam and people are marking them as Spam, then it will have a poor reputation. IPs with poor reputations often encounter issues when sending to Yahoo.
When obvious Spam makes it into the Inbox, it's usually because spammers found a way to be quicker than our system or to exploit a loophole. In these cases, mark the message(s) as Spam. If it keeps happening over a period of days, scan your device for any malware/spyware and let us know so we can report it to our Antispam team.