<p>One of my friends received the following error message when trying to send/receive e-mails in Microsoft Outlook.</p>
<p><strong>The server you are connected to is using a security certificate that cannot be verified.</strong><br />
<strong>The target principal name is incorrect.</strong></p>
<p><img class="alignnone wp-image-3339 size-full" title="The target principal name is incorrect" src="https://www.wincert.net/wp-content/uploads/2019/10/the-target-principal-name-is-incorrect.png" alt="The target principal name is incorrect" width="339" height="164" /></p>
<p>Once he clicked <strong>YES</strong> the warning message went away but as soon as Outlook is restarted he received the same warning message again.</p>
<p>The problem here lies in different mail server name being used from the name the certificate has been issued to.</p>
<p>To solve this click on the <strong>View Certificate</strong> and check the <strong>I</strong><strong>ssued to</strong> value.</p>
<p>Now go to <strong>Microsoft Outlook</strong> and navigate to:</p>
<p><strong>File | Account Setting | Account Setting</strong></p>
<p>Select your <strong>e-mail account</strong> and click on the <strong>Change</strong> button.</p>
<p>Change the values of the <strong>Incoming and Outgoing (SMTP) server address</strong> so it matches the name on the Certificate.</p>
<p>As you might have noticed you probably had a <strong>mail.domain.com</strong> specified for the Incoming and Outgoing (STMP) server addresses which were different from the name the certificate was issued to.</p>
<p>Once you click <strong>Next</strong> and <strong>Finish</strong> the warning message should be gone.</p>

The target principal name is incorrect – Microsoft Outlook
