Fix: Outlook Security Certificate Cannot Be Verified Error [2023]

Getting The Outlook Security Certificate Cannot Be Verified error?

You’re not alone. We had a client ring us this morning to fix up this error. Pretty easy error to fix, once you know what to look for. In our case – it was his clock – wasn’t synced! Do try the other steps though – they are all listed in order of things we would try as per our experience!

So lets talk more about the error – it usually happens when you attempt to connect to an insecure email server, as emails need a digital signature to confirm their legitimacy. A valid certificate is essential for keeping your inbox safe from fraudsters and hackers.

The solution is quite simple: all you have to do is obtain a new digital signature or security certificate from the server or provider. This will update the existing one and restore your connection securely.

To make sure all of your emails are secure going forward, it’s best practice to routinely check that your security certificates are up-to-date.

So Why Does This Error Occur In The First Place? – Causes

The primary cause of this issue is an invalid security certificate. This means that the certificate has a problem or it’s no longer valid, and therefore you won’t be able to connect to Outlook.

Another potential cause is having the wrong date and time on your PC. If the security certificate’s expiration date has passed, then this error may appear.

ISPs also sometimes change the domain name and make changes in the Outlook certificates, so if you are using an old version, that could be causing it too.

Additionally, if you enter an incorrect hostname when adding a new email account in Outlook, then that will prevent you from connecting properly as well.

Lastly, some places such as schools and universities block SSL ports which can also lead to this error because Microsoft Outlook uses these ports for its email accounts.

To avoid these issues with your Outlook certificate, make sure your security certificate is valid and up-to-date, have the correct date and time set on your computer, use only updated versions of Outlook certificates provided by ISP’s, double check any hostnames entered when setting up new email accounts in Outlook for accuracy and make sure none of the SSL ports are blocked at your location.


First, determine if the error is that your machine or server’s clock is not synchronized with the date and time of the certificate itself. If it is, adjust your computer to match the certificate’s date and time settings.

You can also try deleting any previously stored certificates in your browser or Outlook profile. This should help eliminate any mismatched certificates that may be causing a problem.

You should also check for any security software on your device that could be preventing Outlook from verifying certificates, and temporarily disable them as needed.

Finally, you may need to delete or reset the root certificate store in Windows so that Outlook can connect securely again.

1. Verifty Certificate Name

Check if the name on your certificate matches the mail server’s.

Start by clicking “view certificate” when you get a security warning. Then click the “Issue to Name” option and make sure it matches the name of the mail server.

If they don’t match, try changing them and restarting your application. Keep in mind that a single certificate can’t have more than one name, so you may have to check each hostname separately for a solution.

2. Change The Outlook SMTP Port



To do this, follow these steps:

  1. Open Outlook and click on the “File” tab.
  2. Under the “Info” tab, click on “Account Settings.”
  3. Select the account you want to change and click on “Change.”
  4. In the “Advanced” tab, click on “More Settings.”
  5. Change the Outgoing server port to 25.
  6. Click “OK” to save the changes.
  7. Reboot your computer to see if it fixed the error with Outlook’s security certificate not being verified.

3. Disable The Third-Party Apps

To do this, first open Microsoft Outlook and click File, then Options.

Select Add-ins from the left panel, make sure “COM Add-Ins” is selected in the Manage drop-down list at the bottom, and click Go.

Uncheck any faulty add-ins that may be causing your problem and click OK.

Restart Outlook to see if the issue has been fixed.

4. Use Hosting Domain Name As Mail Server

Use your hosting company’s domain name as your mail server.

This will ensure that you always have access to a valid SSL certificate for each domain.

To do this, simply log into your hosting account and change the settings so that all domains point to the same mail server. 

5. Lastly, If Possible – Use An Unsecure Port

Unsecure ports are not as secure as their counterparts, but they can still be used for connecting to mail servers.

This is especially helpful when the SSL POP port 995, IMAP port 993, and SMTP port 465 are blocked on the network you’re connected to in order to protect user privacy and safety. The non-SSL ports that may not be blocked on the same network are: POP 110, IMAP 143 and SMTP 587.

Using these ports will not provide the same level of security that SSL provides but they should still help you connect to your mail server securely with some added precautions from your side like regularly changing passwords and enabling two-factor authentication if available.

Frequently Asked Questions

How do I know if my Outlook Security Certificate is expired?

Check your Outlook security certificate. If it’s expired, you’ll need to renew it in order to access your email securely.

What type of encryption does Outlook Security Certificate use?

Outlook security certificates use Transport Layer Security (TLS) encryption to protect data in transit. TLS provides secure communication between two systems by encrypting and decrypting data. It ensures your data is safe from interception or alteration as it travels across the internet.

How do I make sure that my Outlook Security Certificate is always up-to-date?

You can ensure your Outlook security certificate is always up-to-date by enabling automatic updates. Doing this will keep the certificate current and help protect your data from potential threats.

What is the difference between using a hosting domain name and using an unsecure port for Outlook Security Certificate?

Using a hosting domain name allows you to secure your Outlook security certificate, while unsecure ports can leave it vulnerable. Secure your data the right way and avoid possible errors.

I am a computer engineer holding a bachelor's degree in Computer Science, complemented by a Master's in Business Administration from University of Strathclyde, Scotland. I currently work as a Senior IT Consultant in Melbourne, Australia. With over 15 years of...