A warning is shown while accessing Plesk or webmail: The certificate is not trus

[mahesh]

Question
A warning is shown while accessing Plesk or webmail: The certificate is not trusted or NET::ERR_CERT_AUTHORITY_INVALID
Symptoms
The following error is shown while accessing Plesk or webmail:

• On Google Chrome:

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Your connection is not privateNET::ERR_CERT_AUTHORITY_INVALID

• On Firefox:

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This Connection is Untrusted
(Error code: sec_error_unknown_issuer)
Certificate Error: The security certificate presented by this website was issued for a different website's address. This problem might indicate an attempt to fool you or intercept any data you send to the server.

• On Internet Explorer:

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There is a problem with this website's security certificate.


• On Opera

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Opera cannot verify the identity of the server "XXXX", due to a certificate problem. The server could be trying to trick you
Cause
By default, Plesk is using a self-signed certificate that does not have a signature from the trust center.
Or, the mismatched certificate was applied.

Resolution
Note: If Plesk/WebMail is secured via Let's Encrypt, use hostname/domain name to access it because IP address itself cannot be secured with a valid certificate.

In order to avoid the warning, it is required to have a valid SSL certificate. There are several options on how to achieve that:
Secure Plesk/domain/WebMail via Let's Encrypt:

• For domain/webmail/alias, it is required to go to Websites and Domains > example.com > SSL/TLS Certificates > Install and proceed with the installation. Details are available in the following article:

How to install SSL certificate for a domain in Plesk

• For securing Plesk/Mail Server, go to Tools & Settings > SSL/TLS certificates and proceed from there. See the details in the following articles:

How to secure Plesk login page URL with SSL certificate
How to secure mail server with Let's Encrypt certificate?
Use a purchased SSL certificate:
Purchase an SSL certificate signed for IP address/hostname/domain by a trusted center (Certificate Authority) or install Let's Encrypt extension and obtain a free certificate. See Administrator's Guide - Getting SSL Certificates for details.