Unable to validate the connector from exchange online. Mail flow stoped

Hi @Vergil-V

Thank you so much for your help and response,

As I found the cause. Email hit on another connector due to misconfigured in IP ranges.

Removed IP ranges from same connector after that it worked for default incoming connector on Exchange Server.

Hi @Radhe Chauhan 
Thank you for reaching out to Microsoft Learn Q&A!    

Based on your description, I understand that you're encountering error codes 550 5.4.317 (Message expired) and 451 5.7.3 (STARTTLS is required to send mail) during the email validation step while setting up the Exchange Online connector with TLS enabled and using a certificate that includes Subject Alternative Names (SAN). 

These error codes typically indicate issues with TLS configuration or certificate validation. Based on my research, here are some areas you may want to review:
1.Review Exchange Online Connector configuration: 

• Ensure the connector is set up for the correct mail flow: from Office 365 to your on-premises email server. 
• Confirm that the Subject Alternative Names (SANs) specified in the connector match those listed in your on-premises certificate. 

2.Verify on-premises certificate:
You can verify the certificate's validity and ensure that the Fully Qualified Domain Names (FQDNs) listed in the Subject Alternative Name (SAN) field are correct.
To do this, you can use the Get-ExchangeCertificate command in Exchange PowerShell, which will display details such as the subject, certificate domains, and services. For example:  

Get-ExchangeCertificate | fl Subject, CertificateDomains, Services 

3. Check Receive Connector settings:
You should also review the Receive Connector on your Exchange server to ensure that it is correctly bound to the certificate and that TLS is enabled.
You can use the Get-ReceiveConnector command in Exchange PowerShell to check the TLS certificate name and authentication mechanism. For example: 

Get-ReceiveConnector -Identity "<yourReceiveConnectorName>" | fl TlsCertificateName, AuthMechanism 

4.Firewall or load balancer restrictions:
Confirm that there are no restrictions (such as port 25 being blocked) between Exchange Online and your Exchange Server. 
Reference: Network ports for clients and mail flow in Exchange | Microsoft Learn 

Additionally, if you're working in a hybrid environment and have recently installed or renewed your certificate, it's recommended to run the Hybrid Configuration Wizard (HCW) to ensure that the hybrid setup continues to function correctly and that all necessary configurations are properly updated. 

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"cannot connect to remote server"

Why cant it connect? Was it working before? Does your firewall allow access from ExO?