There are a few reasons this can happen:
- The sender is also on your allow list. The allow list takes precedence over the block list to ensure no desired mail is lost. Check your allow list to make sure the sender is not on it.
- Your automatic junk mail filtering is turned off. Block/Allow list capabilities require Automatic Junk Mail Filtering to be enabled. How do I add an email address to my "blocked senders" or "allowed senders" list with O365 Spam Filtering?
- The servers take up to a half-hour to sync changes made to your Allow and Block lists. If you very recently added somebody to your list, they may still be able to send you messages for up to an hour.
- The person you tried to block has an @mit.edu email address. O365 does not allow internal accounts to be blocked, so anybody with an MIT email address can not be blocked using Spam filtering. You can add them to the list, but it will be ignored. However, you can set up a filter to automatically move messages from any senders from your inbox to other folders.
- The person you tried to block is in your Contacts list. There is a setting in OWA > Options > block or allow to "Trust email from my contacts." Having this enabled will override any additions you may make to your blocked senders list. Uncheck this option to resolve.