That menu option can vary depending on the exact version and modules enabled in ISPConfig. If you don't see Tools -> Resend Mail, here are the quickest native ways to trigger a test email directly through the ISPConfig interface without touching the command line:
Option 1: Send a Client Password Reset Email
Go to Client in the top navigation bar.
Click on any existing Client (or click Add new client to create a quick test user with your personal email address).
Open the client record, type a new password or leave the password field ready, and check the box for Send Password Mail (or click the Send reset password email button if available).
Click Save. ISPConfig will immediately send an automated system message containing the login credentials to the email address assigned to that client.