There is a problem with the proxy server's security certificate

You may receive the error message There is a problem with the proxy server’s security certificate when Outlook tries to connect to a server by using an RPC connection or an HTTPS connection. In this post, we will identify the potential causes, as well as offer the most adequate solutions to resolve this issue.

When you encounter this issue, you’ll receive the following full error message;

There is a problem with the proxy server’s security certificate.
The name on the security certificate is invalid or does not match the name of the target site webmail.domain.com.

Outlook is unable to connect to the proxy Server. (Error Code 0)

Possible variants of the error message with accompanying error code when the error occurs can be any of the following;

There is a problem with the proxy server’s security certificate, %s. Outlook is unable to connect to this server.
There is a problem with the proxy server’s security certificate, %s. The name on the security certificate is invalid or does not match the name of the site. Outlook is unable to connect to this server.
There is a problem with the proxy server’s security certificate, %s. The security certificate is not from a trusted certifying authority. Outlook is unable to connect to this server.

According to Microsoft documentation, this issue may occur if one or more of the following conditions are true:

The connection to the server requires a certification authority (CA).
You haven’t trusted the certification authority at the root.
The certificate may be invalid or revoked.
The certificate doesn’t match the name of the site.
A third-party add-in or a third-party browser add-in is preventing access.

There is a problem with the proxy server’s security certificate – Outlook error

If you’re faced with this issue on your Windows 11/10 PC, you can try our recommended solutions below in no particular order and see if that helps to fix the Outlook error message There is a problem with the proxy server’s security certificate.

Check Proxy server certificate
Install the trusted root certificate
Disable third-party add-on in Outlook
Disable third-party browser add-ins
Manually configure Exchange Proxy Settings in Outlook

Let’s take a look at the description of the process involved concerning each of the listed solutions.

1] Check Proxy server certificate

This solution requires you to check the certificate, and then contact your system administrator to resolve this There is a problem with the proxy server’s security certificate issue.

To check the proxy server certificate, do the following:

Open Edge browser on your Windows 11/10 PC.
Type or copy and paste the URL below into the address bar of the web browser and hit Enter. Replace the server_name placeholder with the RPC server name or the secure server name.

https://www.server_name.com/rpc

Next, click the padlock icon on the address bar.
On the flyout, click Connection is secure.
Click on the Security Certificate icon to view the Security Certificate.
In the Security Certificate properties sheet, click the Details tab.
Now, make a note of the information in the fields highlighted on the image above.

The Valid to field should indicate the date until which the certificate is valid. The data in the Subject field should match the site name – and if this is not the case, contact your IT administrator.

2] Install the Trusted Root Certificate

This solution requires you to install the Trusted Root Certificate on the Windows 11/10 client computer.

Do the following:

When the error occurs and you’re prompted with the Certificate dialog box, click Install Certificate.
Click Next.
Click to select the Place all certificate in the following store check box.
Click Browse.
Click Trusted Root Certification Authorities.
Click OK.
Click Next.
Click Finish.
Click OK.

3] Disable third-party add-on in Outlook

Another viable solution to fix the error in view is to disable third-party add-in in Outlook. To do this, you first need to start Outlook in Safe Mode, and if Outlook successfully opens in Safe Mode, then it’s likely the error is triggered by third-party add-ons.

To disable third-party COM add-ins in Outlook, do the following:

Start Outlook in Safe Mode.
On the File menu, click Options > Add-Ins.
In the Manage box, click COM Add-ins > Go.
Now, uncheck the box for the third-party add-ins that you want to disable.
Restart Outlook.

4] Disable third-party browser add-ins

Depending on your browser, this solution requires you to disable third-party browser add-ins and see if that helps to resolve the issue. Otherwise, try the next solution.

5] Manually configure Exchange Proxy Settings in Outlook

To manually configure Exchange Proxy Settings in Outlook, do the following:

Open Microsoft Outlook.
Navigate to Account Settings > More Settings > Connection > Exchange Proxy Settings.
Now, type in the address below into the Only connect to proxy servers that have this principal name in their certificate field.

msstd:webmail.domain.com

Click OK.

Alternatively, to achieve the same result, do the following:

Press Windows key + X to open Power User Menu.
Tap A on the keyboard to launch PowerShell in admin/elevated mode.
In the PowerShell console, type in or copy and paste the command below and hit Enter.

Set-OutlookProvider EXPR -CertPrincipalName:$null

Exit PowerShell when the cmdlet executes.

How do I get rid of security certificate errors?

To get rid of security certificate errors on Windows 11/10, you need to disable this option. Follow these instructions:

Open Control Panel.
Select Internet Options.
Click the Advanced tab.
Scroll down to the Security section.
Under the Security section, uncheck the Warn about certificate address mismatch option.
Click Apply > OK to save the changes.
Reboot your PC.

Why does Outlook say unable to connect to proxy server?

If Outlook says anything about the proxy server, you need to check the internal settings first. In most cases, it appears when Outlook fails to verify the security certificate of the proxy server. If so, you need to follow the aforementioned tips and tricks to get rid of the problem.

Read: Your Outlook account settings are out of date in Mail or Calendar app.

Why am I getting a security certificate warning?

The reason why you might be getting a security certificate warning on your Windows 11/10 PC is due to incorrect date and time. Security certificates are used by web browsers and computers to make sure a particular site is safe. So, if your computer has the wrong date and time, it can cause certificates to appear invalid & your web browser will begin to issue security warnings. So, make sure the date and time is correct on your computer.

That’s it! Hope it helped.