We've seen a significant improvement in delivery rates and confident this is now resolved. Any deferred emails should resend again shortly.

Microsoft have declared an active incident (EX1467029).

Microsoft is experiencing delivery issues, we're only seeing this in the New York region currently.

Customers may experience elevated 451 4.7.500 Server busy. Please try again later from {IP_ADDRESS} errors.

We will continue to monitor and have opened a case with Microsoft.