The Unauthorized License Detected warning should show up if you are using an illegal version of eM Client, which is detected by your System checking eM Client's program file signatures. Recently, some users encountered this warning even with a valid program and license, because of an outdated certificate in their device.

In the current versions released after January 2023, eM Client now uses Azure Code Signing to sign its program files. The Windows version of the app is signed using a Microsoft certificate to prove its validity and safety when used on Windows devices.
So if you encounter this warning, please make sure to have the latest Windows update on your device installed.

If you do have the latest updates, you can check and/or install the missing certificate directly.

Go to the eM Client installed program files, default C:\Program Files (x86)\eM Client.
Find MailClient.exe and open its Properties.
Open the Digital Signatures tab.
Click the eM Client s.r.o. signature in the list and open Details.
Check the signature here.
If it's shown as invalid, select the Microsoft signature at the bottom and open its Details.
Then click View Certificate button and check the Certification path.
if there is any Microsoft certificate missing (a red X over the certificate icon), install it or add it to trusted certificates.

You can find Microsoft certificates in their repository here: https://www.microsoft.com/pkiops/docs/repository.htm
Search for the name of the missing certificate, download it, and when installing it make sure to select the folder (store) manually and save it into Trusted Root Certificates Authorities.

That should resolve the issue - restart your device and start eM Client again.

For eM Client 9.2.1222 and older you can use this solution:

To check if you are affected by this problem, please check this webpage: https://trusted-root-g4.chain-demos.digicert.com/
If the page shows a warning, you need to update your root certificate DigiCert Trusted Root G4.

This can be sorted by installing available security updates for your Windows or you can find the updates manually here: Support for urgent Trusted Root updates for Windows Root Certificate Program in Windows

You can also manually download the certificate here: 

DigiCert Root Certificates | DigiCert.com

Direct download for DigiCert Trusted Root G4 | DigiCert.com

To avoid similar issues in the future, make sure you have the Windows Root certificate AutoUpdater active.
This can be changed in Windows registry directly in HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\SystemCertificates\AuthRoot
Just make sure the DisableRootAutoUpdate key is set to 0.

If the issue is not resolved even after manually updating or re-installing the missing certificates, you can reinstall the app in case the issue was in the corrupted program files.
Follow these steps:

- Uninstall eM Client (do NOT delete your database)
- Make sure the eM Client folder and files are gone from C:\Program Files (x86)\eM Client or C:\Program Files\eM Client
If not, delete them manually.
- Then download and install the app for your licensed version from an official file downloaded from our website: https://www.emclient.com/release-history