When configuring the SMTP settings within your Endpoint Security Console for Gmail, the Test Email may not be able to be sent. This is due to a security setting within the Gmail account being used for the Endpoint Security Console.
To resolve this issue, follow these steps:
1. Sign in to your Google account
2. From the Home page, go to Security.
3. Scroll down to Less secure app access, click Turn on access
4. In the next page click to Allow Less Secure Apps to ON
Once done, then try sending a test email.