Starting at about Aug 18 10:40 PM ET / Aug 19 2:30 AM GMT, SpamHaus RBL was blocking a number of IPs incorrect.  Those block were removed at various times for the affected systems.

Especially if you use Office365 and were not using a Partner Connector, Office365 would reject mail with:

550 5.7.1 Service unavailable, Client host [] blocked using Spamhaus. To request removal from this list see AS(1440) (in reply to RCPT TO command))

Affected Systems:


cloudp and (but not (but not cloudj or (but not


We have confirmed that all SpamTitan Cloud systems are no longer being blocked after 5:15 AM ET US / 9:15 AM GMT on Aug 19th.

Also, we have gotten reports of non-SpamTitan Cloud IPs starting to be blocked around the same time, so it is very possible that it was something systematic on SpamHaus's end, and subsequently corrected.