Everyone in organisation not receiving emails and external senders getting error message

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Hi @Eleanor Sanders white   

Based on what you shared, the behavior you are experiencing appears to be related to port 25. I would suggest starting with the checks below to help narrow down. 

First, please confirm whether your Exchange server is listening on port 25 by running: 

Test-NetConnection -ComputerName localhost -Port 25  

If the result is True, then the service is listening correctly. In that case, the next step would be to review your Firewall or Router configuration to ensure everything is open and consistent: 

• Inbound TCP port 25 is allowed from any source IP 
• No recent rule changes that may impact connectivity 

If the result is False, then it would be helpful to check the following areas on the server side: 

Transport services status: 

Get-Service MSExchangeTransport, MSExchangeFrontEndTransport  

Receive connector status: 

Get-ReceiveConnector | Select Name, Enabled, Bindings  

Windows Firewall rules related to port 25: 

Get-NetFirewallRule | Where-Object {$_.LocalPort -eq "25"} | Select DisplayName, Enabled, Direction, Action  

I hope this gives you a clear direction on where to look next. If you have any updates or findings, please feel free to share them here 

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