As part of its filtering criteria, the Outlook Junk Email Filter checks message senders against lists of email addresses and Internet domains designated either as safe or to be blocked.
When you no longer want to see messages from someone, you can block a sender.
- Right-click a message from the sender you want to block.
- click Junk > Block Sender.
The blocked person can still send you mail, but if anything from his or her email address makes it to your email account, it’s immediately moved to the Junk Email folder.
It’s a good idea to periodically review email messages identified as junk to make sure you’re not missing important messages.
Source: https://support.office.com/en-us/article/block-a-mail-sender-b29fd867-cac9-40d8-aed1-659e06a706e4