We are currently working on an issue affecting a few of our customers sending mostly to outlook.com addresses, who are receiving bounces due to a spamhaus block.
UPDATE: The spamhaus block is no longer affecting users and we’re continuing to monitor the situation.

When was the spamhaus block applied?
At some point over the weekend – our internal RBL monitor didn’t pick it up because we actually changed our outbound egress IP – we’ve fixed that as well, or we would have seen it right away.