Occasionally, you may run into the scenario where your license key is no longer synchronized with GFI Accounts, thus causing GFI MailEssentials to stop working.
Navigate to General Settings > Licensing, enter your GFI Account username and password, and click on the Login button. After you have logged in, choose your license key from the list of Available Keys.
Alternatively, you can click on the Manually Enter License Key link and enter your license. Please note that the license key is case-sensitive.