I'm in process to migrate my users mailboxes from Exchange 2007 to O365.
I set a Send Connector rule to send all emails for migrated recipients to O365 MX server. Now I got a msg from O365 like:
m #550 5.7.1 Service unavailable, Client host [x.x.x.x] blocked using Spamhaus. To request removal from this list see http://www.spamhaus.org/lookup.lasso
Why my Hub Transport is considered as spam? How can I whitelist my server in O365?