When an account blocks a contact, the system automatically blocks two things:
The contact (user)
The IP address the user was using when they wrote in
This is important because an IP address can be shared. For example, if the blocked contact was writing from an office network, coworking space, hotel, or airport Wi-Fi, the same IP may be used by many other people. As a result, anyone trying to open a new conversation from that same IP address may also be blocked, even if they are different users.
How to unblock an IP address
To unblock the IP address, you must unblock the contact you previously blocked. Once the contact is unblocked, the IP block is removed and chat access/message sending should work again for users on that IP.