Outage in Hornetsecurity
Deferred emails to Microsoft Tenants
Resolved Minor
June 02, 2026 - Started 19 days ago
- Lasted about 8 hours
Incident Report
Summary AI Generated
Hornetsecurity experienced email delivery issues for 8.2 hours where emails to German Microsoft 365 tenants were being deferred due to Microsoft's connection limits being exceeded. The incident affected both inbound and outbound email services, with emails placed in a deferred state for retry delivery (up to 7 days for inbound, 24 hours for outbound). Microsoft resolved the underlying connection limit issue on their infrastructure.
We are currently observing reports of emails being deferred when the receiving Host is a Microsoft Tenant. From our current findings this seems to only affect German M365 Tenants.
The behavior can be identified by following SMTP Code:
421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel
Affected emails are in the deferred state. We are trying to deliver inbound deferred emails for up to 7 days and outbound emails we are trying to deliver up to 24 hours.
Other services are not affected.
The behavior can be identified by following SMTP Code:
421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel
Affected emails are in the deferred state. We are trying to deliver inbound deferred emails for up to 7 days and outbound emails we are trying to deliver up to 24 hours.
Other services are not affected.
Trusted by 1,000+ teams
The Status Page Aggregator with Early Outage Detection
Stop finding out about outages from your users. Monitor 6,320+ cloud services and get alerted the second something breaks.
Latest Updates ( sorted recent to last )
19 days ago - at 06/02/2026 02:48PM
We are currently observing reports of emails being deferred when the receiving Host is a Microsoft Tenant. From our current findings this seems to only affect German M365 Tenants.
The behavior can be identified by following SMTP Code:
421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel
Affected emails are in the deferred state. We are trying to deliver inbound deferred emails for up to 7 days and outbound emails we are trying to deliver up to 24 hours.
Other services are not affected.
19 days ago - at 06/02/2026 03:32PM
Microsoft is working to resolve the observed issue as soon as possible.
More Details can be found here:
https://admin.cloud.microsoft/#/servicehealth/:/alerts/EX1331830
We are monitoring the situation closely.
Other services are not affected.
Latest Hornetsecurity outages
The Status Page Aggregator with Early Outage Detection
With IsDown, you can monitor all your critical services' official status pages from one centralized dashboard and receive instant alerts the moment an outage is detected. Say goodbye to constantly checking multiple sites for updates and stay ahead of outages with IsDown.
Start free trialNo credit card required · Cancel anytime · 6320 services available
Integrations with
