If you are an Exchange Online Protection (EOP) user and you have a 3rd party service in front of Office 365, Rate-Limits may be applied to your SpamTitan IP(s) on the Office 365 mail servers.This is due to the fact that these IP’s are new to Microsoft and do not yet have a reputation. You may notice deffered mail with the reply said 451 5.7.(500-699) Server Busy. Please try again later . This relates to Office 365 IP throttling.
The solution to fix this would be to setup a connector to apply security restrictions to mail sent from your partner organization (or on-premise device) to Office 365. Please find the detailed solution in the following KB article: