Fix Microsoft 365 Internal Emails Cannot Send or Receive

Check your Outlook inbox for a bounce-back email titled 'Delivery has failed to these recipients or groups'.

Read the body of the message to find the exact error description. Look for statements like 'the email system isn't accepting connection requests from your email system'.

Since this often indicates a server-side rejection, contact your recipient through another communication method (like Teams or phone) and ask them to provide these error details to their email administrator.

Log in to the Microsoft 365 admin portal with administrator credentials and navigate to the Exchange admin center (EAC).

In the left navigation menu, go to 'Mail flow' and select 'Message trace'.

Enter the sender's and recipient's email addresses, specify the time range, and click 'Search' to see exactly where the internal email was dropped or blocked.

In the Exchange Admin Center, navigate to 'Mail flow' > 'Rules' to check if any active transport rules are mistakenly dropping or quarantining internal emails.

Go to 'Mail flow' > 'Accepted domains' to ensure your organization's domain is correctly configured as Authoritative or Internal Relay.

Navigate to 'Mail flow' > 'Connectors' and validate that no third-party spam filtering connectors are improperly routing or dropping internal traffic.

If the issue cannot be isolated after checking these settings, have your administrator open a formal support request with Microsoft 365 support for further backend diagnostics.