Solution

Check SMTP settings, enable less secure apps, and unlock Captcha:

1. Confirm the form’s SMTP settings are correct.
2. Enable access to Less secure apps.

How To Enable Email Sending In Gmail?

1. Before sending emails using the Gmail's SMTP Server, you to make some of the security and permission level settings under your Google Account Security Settings.
2. Make sure that 2-Step-Verification is disabled.
3. Turn ON the "Less Secure App" access or click here.
4. If 2-step-verification is enabled, then you will have to create app password for your application or device.
5. For security measures, Google may require you to complete this additional step while signing-in. Click here to allow access to your Google account using the new device/app.

Note: It may take an hour or more to reflect any security changes