We recently uncovered that there are throttling rules on all inbound connectors except “your organization’s mail server”. In order to resolve the issue where messages will queue up on the sender side due to EOP with message
Deferred: 451 4.7.500 Server busy. Please try again later from [x.x.x.x ]. (AS780082)where x.x.x.x is the IP of the sending server to EOP.
Perform the following steps:
1. Click on mail flow, connectors
2. Click the + sign to create a new connector
3. Choose from: your organization’s mail server
4. To Office365
6. Click Next
7. Name it something meaningful
9. Click Next
11. Select Verify by IP address, and add in the SpamStopshere ranges one by one
12. Click Next and the connector will be enabled.
I hope this helps someone else in the middle of the night to avert any major issues!