Relay via Office 365 stopped working - Client host blocked using Spamhaus

Issue

Relaying email via Office 365 suddenly stopped working. Relayed emails were sent from a Windows 2012 Server using a simple PowerShell script. After logging in to the server and running the PowerShell script in an interactive mode following error was displayed:

Exception calling "Send" with "1" argument(s): "Mailbox unavailable. The server response was: 5.7.1 Service unavailable, Client host [212.159.94.214] blocked using Spamhaus. To request removal from this list see http://www.spamhaus.org/lookup.lasso
(AS16012612) [VE1EUR01FT026.eop-EUR01.prod.protection.outlook.com]"

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