After speaking with O365 support we've learned about a new connection filtering policy in test. In order to override the default connection limits, you must override the default policy depending on your mail flow volume
Update 10/01/2020 17:17 PM 18 days
Customers on O365 will have to raise a case with Microsoft detailing the following error rejection codes
Microsoft has started to throttle deliveries from ExchangeDefender with no rhyme or reason. We've opened up a case with Microsoft, but we urge partners to do the same.
Update 10/01/2020 16:56 PM 18 days
We are again seeing random delivery failures and server busy errors while delivering mail to Office365. It appears to be affecting multiple clients.
The issue has been reported to Microsoft and we hope they sort it out quicky (The error they are reporting is (Deferred: 451 4.7.500 Server busy. Please try again later from [65.99.255.116]. (S77719) [CO1NAM04FT)
In the meantime, this is why you have LiveArchive - you can continue to send and receive mail and access all the inbound mail that may be backed up. You can access LiveArchive at https://livearchive.exchangedefender.com