A company's mail server (Postfix) is rejecting incoming emails from a trusted partner with the error '550 5.7.1 Service unavailable; Client host [203.0.113.50] blocked using zen.spamhaus.org'. The partner's IP is not listed on any public DNSBL. What is the most likely cause?
Select one:
The trap here is that candidates assume the error message is literal and the block is from zen.spamh...