It took six days after I detected an outgoing mail Denial-of-Service for Microsoft to publish a public admission that a problem did truly exist. In the contemporary fast-paced IT world, for any problem to take six days to recognize is like waiting to be taken across the river Styx. But, I doubt that Microsoft was working on it's obituary.
Currently Office 365 outbound email servers have a SenderBase reputation of neutral and a score of 0. As a result any policy set to throttle or reject mail from a server rated neutral or with a score of less than or equal to 0 may impact delivery of the mail from Office 365 customers.
Microsoft currently believes this is due to an instance where a large number of bulk mail messages were sent to a user via a server that contributes reputation information. This mail did not get classified as spam by us, the sender is reputable, but the volumes, combined with Cisco’s rating system, have temporarily reduced our email servers' reputation in their SenderBase service. According to Cisco, it will take time and additional mail flowing through their system to retrain it and restore our email servers’ reputation.