Spark post claims to have found and corrected the issue.
We sincerely apologize for the delay in responding.
We recently identified an anomaly in our systems that affected our overall sending performance to Microsoft domains. This issue has now been resolved by our engineering team, however, messages that bounced will need to be retried for sending.
Please accept our apologies for any inconvenience this may have caused and let us know if you have any additional questions.
Regards,
SparkPost Compliance