To decrypt StealthMail Emails you will need a StealthMail Add-in for Outlook or any other StealthMail Client Application for Windows, macOS, Android or iOS.
Download it, install, add your Email account and register StealthMail account in it. Click on a Stealth-link in received secure email. StealthMail will open and decrypt your secure Email.
StealthMail Add-in manual
for Microsoft Outlook