Delayed email delivery to Microsoft 365 and Outlook recipients
Since approximately 09:00 CEST on 4 September 2026, we are seeing delays in the delivery of outgoing email to recipients whose mailboxes are hosted at Microsoft — Microsoft 365 / Exchange Online, as well as outlook.com and hotmail.com.
Microsoft's mail systems are currently responding to a large number of senders worldwide with a temporary "Server busy" message (SMTP 451 4.7.500). Affected messages are therefore not delivered immediately, but are queued and retried automatically. Microsoft has confirmed the fault under service advisory EX1467029 — "users may experience delays sending and receiving email messages from external domains" — published on 4 September 2026 at 11:03 UTC, and is working on a resolution. Customers who operate their own Microsoft 365 tenant can view this advisory under Health → Service health in their Microsoft 365 admin center.
What this means for you
- Affected messages are not lost. They remain in our mail queue and are retried automatically until Microsoft accepts them.
- Delivery to all other providers is unaffected.
- Incoming email and all other hosting services are operating normally.
What you should do
- No action is required on your part.
- Please do not resend affected messages repeatedly. Additional copies increase the load and can prolong the delay.
What we have done
- Extended the retry period, so messages are still delivered once Microsoft's systems recover.
- Adjusted our delivery rate towards Microsoft to reduce the impact.
- We are monitoring continuously and will update this notice as soon as we have further information.
The cause lies within Microsoft's infrastructure and is outside our control. We apologise for the inconvenience.
