Update 10/01/2020 16:56 PM 2 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 [126.96.36.199]. (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