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

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