Microsoft Exchange Online Outage

As delivery to Microsoft 365 has become more reliable again, we have now lifted the temporary restrictions on our German cloud infrastructure. Messages from and to Microsoft 365 are being accepted and processed normally again.

The backlog that built up during the disruption is being processed. The number of queued messages on our systems has already been reduced by around 80 percent, and the remaining messages are being delivered continuously.

What this means for you:

• Messages that were temporarily rejected by our systems are still queued at Microsoft 365 or at the sending server and will be retried automatically.
• No action is required on your side.
• Some messages may still arrive with a delay until the remaining backlog has been processed.
• If Microsoft 365 was unable to retry certain messages for an extended period, senders may have received non-delivery reports (NDRs). In this case, we recommend resending the affected messages.

We continue to monitor the situation closely and will provide a final update once the backlog has been fully processed.

As the Microsoft 365 service disruption is still ongoing (see https://status.cloud.microsoft/), we have had to take a temporary protective measure on our German cloud infrastructure.

For customers using parallel mode, our gateways are currently not accepting messages from Microsoft 365 servers and respond with a temporary error instead. Because Microsoft is currently unable to reliably accept mail, messages we process could not be handed back to Microsoft 365 for delivery. Without this measure, these messages would accumulate on our systems and put the stability of the entire platform at risk.

What this means for you:

Affected messages remain in the Microsoft 365 queue and are retried automatically. No messages are lost, and no messages bypass SEPPmail processing.
Outgoing mail will be delayed until the measure is lifted.
If the Microsoft disruption continues for an extended period, Microsoft may return non-delivery reports (NDRs) to senders for messages it can no longer retry.

We are aware that this is an exceptional emergency measure. We are closely monitoring the situation and will lift the restriction as soon as delivery to Microsoft 365 is reliable again. We will inform you when normal operation has resumed.

As reported by several news sources (ex https://www.heise.de/news/Microsoft-365-Stoerung-Serverausfaelle-legen-Teams-Exchange-und-Copilot-lahm-11523565.html) Microsoft is experiencing massive issues. Due to that we’re seeing a lot of mail that could not be delivered by our systems to Microsoft server yet.

If your seeing your outoging message to Microsoft recipients deferred by messages like:

421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel

or

421 4.3.2 The maximum number of concurrent server connections has exceeded a limit, closing transmission channel

or

451 4.7.0 Temporary server error. Please try again later. PRX3 NextHop

then your mails are affected by the issue. There is nothing SEPPmail can do. We have to wait until the situation normalizes on Microsoft’s end.

Outgoing mail to Microsoft recipients via SEPPmail cloud will not be lost but queued in our fallback systems which will try to re-transmit the mails in regular intervals