Can somebody please tell me how can I un-block my friend to get his messages in my mailbox once again?

Go to your इनबॉक्स and select the Blocked Users section. There should be a green Unblock button to the right of your friend's username. Click on that button and you've unblocked him.

If you're on फैन्पॉप mobile site, then आप should go to your mailbox using the यू. आर्. एल with a tag /messages/trash आप can find all of the messages sent to आप द्वारा your friend right there, amongst your trash mail. All आप gotta do now is simply to choose your friend's message & tap Undelete selected messages button beneath the list. That's it. It's as simple as that! Your friend is now unblocked & all of his messages will go to your इनबॉक्स instead of trash. Hope it helps. It works for Android 6.0 या higher. I'm not sure about the other mobile OS's though. आप forgot to mention what kind of a mobile OS do आप actually use, if you're using the फैन्पॉप mobile site in the first place. आप should be और specific in the future when it comes to और detailed informations, so that we can all provide आप with an adequate & satisfactory answer आप deserve.