Been trying to get Office 365 support to help but so far no luck.
Last week one of our users could not send to specific clients. Now this week another user is having the same problem. Running a message trace on the failing messages gives the message:
Reason: [{LED=550 5.7.501 Service unavailable, Client host blocked using Spamhaus. To request removal from this list see http://www.spamhaus.org/lookup.lasso (AS16042849)};{MSG=};{FQDN=};{IP=};{LRT=}]
When I look up the IP address within the Message Trace it is indeed listed in Spamhaus. I haven't gotten much help with this yet due to the fact Office 365 support says the IP is not owned by Microsoft, which appears to be correct as it's listed as being
owned by AT&T. However I don't see how this is anybody else's issue but Microsoft's as Office 365 handles all incoming/outgoing email for the tenant.
I've had a ticket open for almost two weeks, but have made no progress thus far. We just recently migrated to our entire company to Office 365 and are now dealing with this. Very frustrating!
Has anybody else had to deal with this? So far I've encountered two IP blocks that Microsoft uses that appear to be owned by AT&T that is currently listed in Spamhaus.