Cannot Send Email From Exchange Online (365) To Exchange Server (On-Premise)

Question

Hi, does anyone has a solution for my issue in the hybrid exchange environment, I can't send email from 365 to Onprem user. This is the error in the message trace

Thank you.

Answer 1

Hi @Ardan Zaki ,

Typically, this error means Microsoft 365 connected to the destination email server, but the server responded with an immediate error, or doesn't meet the connection requirements. The error details will explain the problem. For example:

• The destination email server responded with a "Service not available" error, which indicates the server is unable to maintain communication with Microsoft 365.
• The connector is configured to require TLS, but the destination email server doesn't support TLS.

Please verify the TLS settings and certificates on your on-premises email servers, and the TLS settings on the connector.

In addition, please check is there anything between Exchange and O365, for example firewall,  antivirus software, or any other third-party software installed on Exchange that may interfere. 

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Answer 2

Have you enabled TLS 1.2 on your on-prem Exchange Servers?

https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-server-tls-guidance-part-1-getting-ready-for-tls-1-2/ba-p/607649