Fix: Outlook Security Certificate Cannot Be Verified Error

The “The server you are connected to is using a security certificate that cannot be verified” error in Outlook usually appears when Outlook connects to a mail server or Autodiscover endpoint whose certificate does not match the hostname being used, or when the certificate chain is not trusted.

In most cases, this happens because the incoming or outgoing mail server name is incorrect, the hosting provider uses a different secure mail hostname than your custom domain, or Outlook is being redirected to an Autodiscover address that is not included in the certificate.

Below are the fixes that usually solve the problem, starting with the most effective ones.

1. Check the Certificate Name and Match It to the Mail Server

This is the most important check. If Outlook is connecting to a hostname that is not listed on the certificate, the warning will continue to appear until you use the correct server name or the certificate is replaced.

1. When the warning appears, click View Certificate.

   

2. Check the Issued to or certificate name shown in the certificate window.
3. Compare that name with the incoming and outgoing mail server names configured in Outlook.
4. If they do not match, update Outlook so it uses the hostname that matches the certificate, then test the account again.

Example: if the certificate is issued to mail.hostingcompany.com, but Outlook is configured to use mail.yourdomain.com, Outlook may reject the connection until you switch to the hostname covered by the certificate.

2. Verify the Exact Mail Server Settings with Your Email Provider

Do not guess the server names, ports, or encryption method. Even if the username and password are correct, Outlook can still trigger this warning if the account is pointing to the wrong server or using the wrong encryption settings.

1. Open Outlook.
2. Go to File > Account Settings > Account Settings.
3. Select the affected account and click Change.

4. 

5. Verify the following with your provider or hosting company:
   • Incoming mail server name
   • Outgoing mail server name
   • Account type: IMAP or POP
   • Port numbers
   • Encryption method: SSL/TLS or STARTTLS
6. Click More Settings > Advanced, correct the values if needed, then click OK.

7. 

Important: avoid switching to random “fallback” ports just to suppress the warning. If the server name and certificate still do not match, the real issue remains unresolved.

3. If You Use Shared Hosting, Use the Hostname That Matches the Certificate

This problem is common on shared hosting. Many hosting companies secure mail using their own hostname, not your custom domain. If Outlook is configured to use your domain but the certificate is issued to the host’s mail server name, Outlook will show a certificate warning.

In that case, use the secure mail hostname provided by your host.

For example:

mail.hostingcompany.com

Instead of

mail.yourdomain.com

If you want to keep using mail.yourdomain.com, your host or server admin must install a certificate that includes that hostname.

4. Fix the Certificate or Autodiscover Configuration if You Manage the Server

If this is a company-managed mailbox, Microsoft 365 hybrid setup, Exchange environment, or self-hosted mail server, the issue may not be on the Outlook side at all. The warning can appear when Autodiscover DNS records or the server certificate are configured incorrectly.

If you manage the server or can contact the admin, ask them to verify:

• the certificate includes the hostname Outlook is reaching
• the certificate chain is trusted
• Autodiscover is pointing to the correct endpoint
• the server is presenting the correct certificate for mail services

If those values are wrong, the warning will keep returning even after you re-add the account.

Note: only install a certificate manually if it comes from your organization, hosting provider, or mail administrator and you know it is intended for that mail service. This is most common with self-signed or private CA environments. For normal public mail services, Outlook should usually trust the certificate automatically when the server name and certificate are configured correctly.