Here’s how:

1. In the upper-right corner of your Mail page, click Options and select Mail Options.
2. Click Spam on the left side of the page.
3. Look for the “Blocked Chat IDs” category on this page.
4. Click in the button beside “Block chats only from those I choose to block”.
5. From the list of blocked contacts that appears, select the contact you want to un-block and click Remove.
6. Click Save.