Exchange Online issues? 421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded

Posted by graphik_@reddit | sysadmin | View on Reddit | 37 comments

Hello reddit,

I am currently seeing a lot connection issues with MS Exchange online:

"The server 52.101.170.2 did not accept the email. The error returned from the remote host was: 421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel. pod72038ehf.outlook.com. PRX8 [Name=InboundProxyTrackingAgent][AGT=IPT][MxId=11BD7F1F8DAC5C5B] [FR3PEPF00000487.DEUP281.PROD.OUTLOOK.COM 2026-06-02T12:25:40.012Z 08DEBF8FC7A9CB94]"

Does anyone else see this too?
I am pretty sure its not my system causing this issue, I do not have a high volume on emails :S

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Proper-Life-143@reddit

+1 Germany

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enzulu@reddit

Just wondered, and also seeing this happen. Good I'm not the only one

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Rumir@reddit

"Ah, good. The problem is not me, but Microsoft"

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Kaneshir0@reddit

this is the way

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LordAkiga@reddit

As always

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Turbojelly@reddit

Probably related to the CoPilot issues.

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SubstantialMall6807@reddit

Same here, across Frankfurt tenants in our email security product.

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15_Tries_All_Taken@reddit

We are in North America and also having delivery delays. Queues starting backing up around 10:22 EST for us.

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Exotic_Ebb_7192@reddit

It's a known issue

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sm_1611@reddit

germany +1

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LeDonCampeon@reddit

Seems to be a bigger Problem: NoSpamProxy Cloud

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Anguel_@reddit

Same reported by Hornetsecurity: https://live.hornet-status.com/

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Different-Group1682@reddit

Same here, German Tenant

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RNK_EDDIT@reddit

Same here in Germany.

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Pirukandji@reddit

Users may be experiencing significant delays in sending and receiving email messages in Exchange Online

Issue ID: EX1331830

Affected services: Exchange Online

Status: Service degradation

Issue type: Incident

Start time: Jun 2, 2026, 4:33 PM GMT+2

More info

Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Scope of impact

Impact is specific to users located in North America and Germany attempting to send and receive email messages in Exchange Online.

Current status

Jun 2, 2026, 4:40 PM GMT+2

We’re analyzing support case details to determine the next steps needed to resolve this issue.

Next update by:

Tuesday, June 2, 2026 at 6:30 PM GMT+2

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UserneT86@reddit

+1 Germany

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deschriever@reddit

same here. some customers are calling us much errors in Mailflow. Germany Tennants

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janmat73@reddit

+1, German Tenant

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DevHopps@reddit

Same here, multiple german tenants.

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War_Sure@reddit

same issue in Germany, we are not alone 😄

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Pirukandji@reddit

Same here, German tenant.

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jtroendle@reddit

Thx for opening this topic, you saved me own research :-) And yes, German/Frankfurt tenant as well

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CommanderCT@reddit

This sucks 🥲

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DaHunni@reddit

Should've looked here earlier...
Same over here

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Few-Fix4781@reddit

I have the same issue but it seems to be limited only to tenants hosted in Germany. To other Microsoft 365 tenants it goes through without any problems.

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graphik_@reddit (OP)

From my analysis I can confirm this. All affected host are containing “fr” in their hostnames which seems to confirm this theory.

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enzulu@reddit

I've checked 3 other hybrid setups for now they are not affected, I've also created a ticket just for ms to hopefully see it.

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Complete-Career8714@reddit

Same issue here; incoming emails don't seem to be delivered either. Tenant is hosted on German servers.

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AlCapone90@reddit

Hey together,

GermanyWest Infrastructure seems to be affected. No information from Microsoft.

Just codetwo is Posting this in their Statuspage:

Statuspage codetwo

Exchange online protection has degradet availability.

WE can reach our mailboxes. But no mail routing possible.

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graphik_@reddit (OP)

Thx, all done already. 👌

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craigleary@reddit

Same, never seen the error before and this was the first and only result.

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t1bb3r@reddit

4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel. pod72038ehf.outlook.com.

450 4.4.318 Connection was closed abruptly (SuspiciousRemoteServerError)};

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t1bb3r@reddit

Me too, just open a ticket to Microsoft