Microsoft is investigating Exchange Online mail delivery issues causing "Server busy" errors and delays when receiving emails from outside organizations. 

According to user reports online, the Exchange Online problems started this morning, affecting Microsoft 365 customers worldwide, across the Americas, Europe, and Asia.

Microsoft confirmed the ongoing issues this morning, at 6 AM GMT+2, saying that "some users may encounter delays receiving external email messages in Exchange Online. Affected users may see a '451 4.7.500 Server busy' error message." 

"We're analyzing sample throttling IPs from simple messages to confirm whether the issue with the portion of SOL infrastructure is causing impact, before we begin formulating a remediation plan," the company added.

"Impact is specific to some users who are served through the affected infrastructure."

More information on these ongoing Exchange Online issues is available under EX680695 in the Microsoft 365 admin center. 

We’re currently working on a fix to address an issue causing delays receiving external email messages in Exchange. Additional details and updates will be provided in the admin center under EX680695.

— Microsoft 365 Status (@MSFT365Status) October 11, 2023

Linked to IP-address anti-spam rules

In a subsequent update, Redmond said the cause of the issues could be linked to the erroneous enforcement of IP address anti-spam rules for affected customers.

"We've identified that a recent service update, applied to a section of infrastructure responsible for enforcing IP address anti-spam rules, contains a change which is inadvertently causing impact," Microsoft said.

This confirms user reports saying that, in some cases, they're seeing thousands of emails added to the outbound queue because of Exchange Online's spam filter.

Microsoft has yet to confirm the regions affected by this Exchange Online outage and if it also impacts Exchange Online outgoing mail delivery.