Outlook 2010 disconnected with TLS 1.2

When my normal laptop died last week I had to use an older laptop, and this laptop had Windows 7 and Outlook 2010 installed, one of my personal favorite Outlook clients.

However, Outlook 2010 did work correctly with Mailboxen in Exchange Online, but Outlook refused to work with Mailboxen on my on-premises Exchange 2016 server. The only thing I saw in the lower right corner was “Disconnected” and every now and then Outlook tried to connect, but no luck.

When checking the Connection Status in Outlook I could see that the directory connection was established, but the Exchange Connections disconnected. The Exchange sever and mailbox were ok since I was able to connect using OWA and my Outlook for iPhone client.

The Test Email AutoConfiguration option in Outlook wasn’t very helpful either, it just showed that it was unable to determine the settings and none of the Autodiscover options worked.

Using the Internet Explorer browser I tried to access my Autodiscover.exchangelabs.nl site, and after a logon prompt I got the famous ErrorCode 600. This is good, so I know my Autodiscover is at least listening properly.

The Exchange Remote Connectivity Analyzer (http://aka.ms/exrca) showed that there was an issue with my SSL certificate:

The SSL certificate however is a valid Digicert UC certificate and there’s nothing wrong with this certificate. IE does use it, and the Digicert help utility doesn’t show anything strange either.

Oh, and my Outlook 2016 running on another computer did work correctly, so there should be a configuration error impacting Outlook 2010 only.

Then I realized that a week before I accidentally ruined the Virtual Service on my Kemp Load Balancer and I quickly created a new Virtual Service using the correct template. As a security measure I only selected TLS 1.2 on the SSL properties of the Virtual Service.

After enabling TLS 1.0 on the Virtual Service, Outlook 2010 started to work correctly again and (to my surprise) so did the Remote Connectivity Analyzer.

So, obviously TLS 1.0 was the culprit here and by enabling TLS 1.0 Outlook 2010 started to work again.

When checking my laptop using the SSLLABS website (https://www.ssllabs.com/ssltest/viewMyClient.html), all looks fine and TLS 1.2 is fully supported by my Windows 7 client:

It must be something with Outlook 2010 and TLS1.2. I found an interesting article on Technet regarding enabling of TLS 1.1 and TLS 1.2. Create a DWORD value DefaultSecureProtocols in the registry under the following keys:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\WinHttp

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\WinHttp

Its value should be one of the following:

For only TLS 1.1 and 1.2: A00 (hexadecimal)

For TLS 1.0, 1.1, and 1.2: A80 (hexadecimal)

Also, create the following DWORD values DisabledByDefault in the following locations and assign it the value of ‘0’:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Client

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client

When needed create the necessary subkeys under the \Protocols key.

Now your Windows 7 and Outlook 2010 will support a TLS 1.2 environment only (this is also true for Windows 8 BTW).

Summary

Outlook 2010 does not support TLS 1.2 out of the box. This can be an issue if you or your network department starts implementing a TLS 1.2 environment only. You have to enable TLS 1.2 on the workstation by setting a registry key. After this it works fine.

Next October Microsoft will stop support for TLS 1.0 and TLS 1.1. This means that if you run into an issue caused by TLS 1.0 or TLS 1.1 it won’t be fixed. Please note that Microsoft will continue to accept TLS 1.0 and TLS 1.1 connection from clients, it just won’t be supported anymore.

Microsoft is working on a plan to disable TLS 1.0 and TLS 1.1 but that won’t happen anytime soon. When this is going to happen, Microsoft will give notification 6 months in advance of disabling TLS 1.0 and TLS 1.1.