We are currently seeing an increase in email delivery delays to Microsoft Office 365 / Exchange Online recipients caused by Microsoft applying IP-based throttling (rate limits).

When Microsoft applies these rate limits, messages exceeding the permitted rate may be temporarily deferred rather than permanently rejected. Affected messages remain queued and are retried automatically, which can result in delayed delivery.

Recommended Action for Office 365 Customers

To help prevent or reduce throttling, we recommend that customers using Exchange Online Protection (EOP) / Microsoft 365 configure an inbound connector in Microsoft 365 for mail received from SpamTitan.

Configuring the connector allows Microsoft 365 to recognize and appropriately handle mail originating from the SpamTitan service rather than relying solely on the reputation of the sending IP addresses.

Instructions are available here:

SpamTitan and Office 365 IP Throttling (Rate Limits)

https://docs.titanhq.com/en/9287-spamtitan-and-office-365-ip-throttling--rate-limits-.html

Our team is continuing to monitor the situation. Customers experiencing delayed messages to Office 365 recipients are encouraged to implement the connector configuration above.