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Understanding TLS Versions for Email Scenarios

Exchange Online: Understanding Email Scenarios If TLS Versions Cannot Be Agreed On
Introduction
Secure email communication is essential for organizations to protect their data and ensure compliance with regulatory standards. As part of this, organizations must ensure that emails sent between their servers and other organizations are encrypted using Transport Layer Security (TLS). TLS is a protocol that ensures that messages are encrypted in transit and are not readable by anyone other than the intended recipient.

It is possible, however, for two servers to be unable to agree on the version of TLS to use when sending emails. This can lead to a scenario where emails sent from one server to another cannot be delivered. In this blog, we will explore what happens in such scenarios, and how to troubleshoot them when using Exchange Online.

What Is TLS?
Transport Layer Security (TLS) is a protocol used to encrypt data sent over a network. It is used by organizations to protect their data by encrypting email messages and other data sent over the internet. TLS is an industry-standard protocol, and is used by most organizations when sending emails.

Why Do TLS Negotiations Fail?
TLS negotiations can fail if the two servers attempting to establish a secure connection cannot agree on the version of TLS to use. This can happen if one server is using an old version of TLS, while the other server is using a newer version.

In such cases, the two servers may not be able to establish a secure connection and the email message may not be sent. This is because both servers must agree on the version of TLS to use for the connection to be established.

What Happens When TLS Negotiations Fail?
When TLS negotiations fail, the email message is not sent and an error is returned to the sender. The error will usually indicate that the message was not sent because the sender and the recipient could not agree on the version of TLS to use.

In such cases, the sender can retry sending the message using a different version of TLS. However, if the recipient server does not support the version of TLS used by the sender, the message will still not be sent.

How To Troubleshoot TLS Negotiations Failures With Exchange Online
When troubleshooting TLS negotiations failures with Exchange Online, the first step is to ensure that the Exchange Online server is using the latest version of TLS. This can be done by using the Get-ExchangeServer cmdlet to check the TLS version of the Exchange Online server.

If the version of TLS used by the Exchange Online server is not the latest version, the version can be updated by using the Set-ExchangeServer cmdlet. Once the version of TLS used by the Exchange Online server has been updated, it should be tested by sending a test message to a remote server.

If the test message is sent successfully, the issue has been resolved. If the test message is not sent successfully, the issue may be due to one of the following:

* The remote server does not support the version of TLS used by the Exchange Online server.
* The remote server is using an old version of TLS.
* The remote server is blocking the Exchange Online server.

If any of these issues are suspected, further investigation is required to determine the cause and resolve the issue.

Conclusion
TLS negotiations can fail if the two servers attempting to establish a secure connection cannot agree on the version of TLS to use. This can lead to a scenario where emails sent from one server to another cannot be delivered. To ensure that emails sent from Exchange Online are delivered, it is important to ensure that the Exchange Online server is using the latest version of TLS. If the issue persists, further investigation is required to determine the cause and resolve the issue.
References:
Understanding email scenarios if TLS versions cannot be agreed on with Exchange Online

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