SMTP 554 5.1.0 Sender Denied NDR

A client of mine was unable to receive any emails from one particular email address. The NDR returned "Sender Denied". Turns out the user had blocked them by accident at some time.

I managed to identify the problem with Get-MailboxJunkEmailConfiguration Jamie which returns the below.

You will see BlockedSendersAndDomains has a single entry in it. This was the email address with the problem.
To remove it, I ran Set-MailboxJunkEmailConfiguration Jamie -BlockedSendersAndDomains @{remove="email@address.com"} and it removed the user.

To confirm that the change has taken effect, run Get-MailboxJunkEmailConfiguration Jamie
You will see there is no entry in BlockedSendersAndDomains