Opt-in to the Exchange Online Endpoint for Legacy TLS Clients using SMTP Auth 

Opt-in to the Exchange Online Endpoint for Legacy TLS Clients using SMTP Auth 

We all know that Exchange online ended support for TLS 1.0 and TLS 1.1 in 2020 to meet the security and compliance requirements. However, Microsoft still allows clients to use legacy TLS versions to connect with Exchange Online. 

In 2022, Microsoft has planned to completely disable older TLS versions in Exchange Online to improve security. If your organization still uses clients that aren’t compatible with TLS 1.2, then the new opt-in endpoint can be set to use the previous versions of TLS.

Opt-in to the Legacy Client Endpoint: 

To use legacy TLS versions, admins need to change the following settings. 

To view the current status of the AllowLegacyTLSClients property, run the following cmdlet. 

Get-TransportConfig | Format-List AllowLegacyTLSClients

To opt-in, execute the below cmdlet. 

Set-TransportConfig -AllowLegacyTLSClients $true
  • Configure clients and devices to use the new endpoint smtp-legacy.office365.com 

 

Opt-out Legacy Client Endpoint: 

admins can opt-out or force TLS 1.2 by checking the TLS usage report for SMTP in Exchange Online.
To disable legacy TLS, admins need to modify the following configurations. 

  • To set –AllowLegacyTLSClients to false, execute the cmdlet as follows.  
Set-TransportConfig -AllowLegacyTLSClients $false
  • Configure clients to accept smtp.office365.com endpoint. 

 

What will happen if you have Missed Opt-in and Still using TLS 1.0 and 1.1?  

If you have not configured to use the legacy endpoint, you will get the following error message.  

421 4.7.66 TLS 1.0 and 1.1 are not supported. Please upgrade/update your client to support TLS 1.2. Visit https://aka.ms/smtp_auth_tls 

The error occurs due to Exchange Online rejecting a small number of TLS 1.0 or 1.1 submissions. 

I hope this blog will help you review and opt-in legacy client endpoint settings. You can share your experience using TLS 1.0/1.1 in the comment section. 

Opt-in to the Exchange Online Endpoint for Legacy TLS Clients using SMTP Auth 

by kathy time to read: 1 min
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