If a user has been blocked because Microsoft's algorithms have determined that they are sending spam, you can unblock them here, but not indefinitely. There's a limit to the number of times an administrator can do this for a specific user; after a certain point, you will get an error, and will have to contact support.
You can also use the Get-BlockedSenderAddress and Remove-BlockedSenderAddress cmdlets in PowerShell to bring back information about a given blocked sender address:
Get-BlockedSenderAddress -SenderAddress smtpaddress@domain.com
And, you can use them to remove the address from blocked senders:
Remove-BlockedSenderAddress -SenderAddress smtpaddress@domain.com
