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

Go to your ইনবক্স 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 ফ্যানপপ mobile site, then আপনি should go to your mailbox using the লিঙ্ক with a tag /messages/trash আপনি can find all of the messages sent to আপনি দ্বারা your friend right there, amongst your trash mail. All আপনি 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 ইনবক্স instead of trash. Hope it helps. It works for Android 6.0 অথবা higher. I'm not sure about the other mobile OS's though. আপনি forgot to mention what kind of a mobile OS do আপনি actually use, if you're using the ফ্যানপপ mobile site in the first place. আপনি should be আরো specific in the future when it comes to আরো detailed informations, so that we can all provide আপনি with an adequate & satisfactory answer আপনি deserve.