Outlook message, "“The server you are connected to is using a security certificate that cannot be verified. The target principal name is incorrect.”

Question

When I open Outlook 365, I repeatedly get the pop-up message, “The server you are connected to is using a security certificate that cannot be verified. The target principal name is incorrect.”

 

I have a Desktop PC and a subscription to Microsoft 365 and I use the Microsoft Outlook 365 (I believe it is the “Classic Outlook”) from my desktop.

I have a POP3 email account with my email provider, SOS4Net, Inc.

 

My email had been working fine until about January 18, 2025 in which time I could not receive email unless I check the “Do you want to continue using this server?”, “Yes” box.

 

Attached is the Screenshot of my email provider’s security certificate from Sectigo RSA Domain Validation Secure Server CA.  I have verified my email account settings with my email provider and everything seems to be good and the certificate looks to be valid. My email provider said they haven’t changed anything.

 

I have no such message popping up issue when accessing my email from my iPhone or iPad Mini and I can receive the email just fine, so the issue seems to be with Microsoft.

Mail Server Certificate 2-2025.jpeg

Answer 1

Looks like that Outlook is detecting a mismatch between the server name you’ve configured in your POP3 account settings and the names listed in the security certificate provided by your email provider. In simple terms, the certificate’s subject (or one of its Subject Alternative Names) does not match the domain name Outlook is using to connect.

Even though the certificate appears valid (issued by Sectigo RSA Domain Validation Secure Server CA), the check on the identifier name is failing.

Some troubleshooting you can do;

1. Server Name Mismatch: The POP3 server name configured in Outlook might not match the common name (CN) or Subject Alternative Name (SAN) on the certificate. For example, if the certificate is issued to mail.sos4net.com but your account is connecting to pop.sos4net.com, Outlook will flag it as a mismatch.
2. Changed DNS Settings or Certificate Renewal: It’s possible that your email provider renewed the certificate or updated their DNS records. Even a subtle change (for example, replacing a “www” prefix, or switching between “pop” and “mail”) can trigger the error.
3. Different Behavior Between Clients: Mobile clients (iOS Mail, for instance) sometimes handle certificate mismatches more leniently or might have cached prior connections differently. Outlook for Windows, especially in the “Classic Outlook” mode, tends to enforce stricter security validations.
4. Cached Settings or DNS Issues: Occasionally, cached DNS entries or Outlook settings can cause an outdated server name to be used, even if the provider’s certificate has been updated.

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