What is the issue?
Email is not delivering to Office 365, resulting in this server response: 451 5.7.(500-699) Server Busy. Please try again later.
What causes the issue?
If you are an Exchange Online Protection (EOP) user and you have a third party service in front of O365 rate limits may be applied to your SpamTitan server IP(s) on the O365 mail servers. This is due to the fact that these IPs are new to Microsoft and do not yet have a reputation.
When rate limits are applied, all mail exceeding the allowed rate will be temporarily rejected. The rejected emails will end up in the deferred queue with the reply 451 5.7.(500-699) Server Busy. Please try again later.
This relates to O365 IP throttling.
What is the solution?
One solution to fix this is to create a connector to apply security restrictions to mail sent from your partner organization (or on-premise device) to O365.
You can also consider the following workout around if the first attempted solution did not work. This involves allowing time for the new IP to be 'learned' by O365 by stipulating more than one MX record, which would include an address directly to their O365 address.
For example, setting MX records as:
This means routing 66% of the mail directly and 33% via SpamTitan.
We do not know the quantity of mail that exceeds this hidden limit, or the exact time for rate limiting to be removed.