I’ve just managed to fix this problem.
I did yesterday all the steps to “Create a Web App in Your Google Account” and it wall worked well, successfully sending the test email.
Today I entered the plug in on my wordpress dashboard and received an error. Then saw that the Client Id and Client Secret boxes were empty.
So went to my google API’s to copy them again. Then received the “redirect_uri_mismatch” error while trying to authorised.
I went back to Google API’s and deleted the Authorized redirect URI.
Then I simply put again the URL there and saved.
Went back to wordpress admin, clicked on the authorization button and finally it worked.
