how to unblock a non-spam sender

This seems to be blocked way before it reaches your mac, so there is nothing you can at your mac to change that.

If the messages are being blocked by your mail provider, contact them, or see if there is some settings you can change by logging in to their webmail interface. What provider is this?

IP addresses are routinely blocked by ISPs if there has been a lot of Spam activity for instance. The IP address is automatically assigned by the ISP according to which server they are uring, but they can change that IP address by simply resetting their router (unplug from wall, wait 30 seconds, plug back in). Wait until all the lights are back on and steady and then test - they should be using a different IP address.

That will not help if a server between your isp and the sender is misconfigured or blocking ip addresses because of spam.

Do what Babowa suggested.

Have a nice day.