Error received during Test Email

I have successfully linked my website with Gmail. I received this success message.

You have successfully linked the current site with your Google API project. Now you can start sending emails through Gmail.

But then when I tried to create an Email Test, I received this error,

An issue was detected.

This means your test email was unable to be sent.

Typically this error is returned for one of the following reasons:

- Plugin settings are incorrect (wrong SMTP settings, invalid Mailer configuration, etc).
- Your web server is blocking the connection.
- Your host is rejecting the connection.

The page I need help with: [log in to see the link]