Google implemented settings to make your gmail more secure. In order for the THUM to send email using a gmail account you need to go through a few extra steps.
First configure the THUM to send email using Gmail as shown below. Entering your own email addresses and account information then click the Test Email button
Once you do that you will receive notification in your Gmail account that a Blocked sign-in occurred.
Click on the Allowing access to less secure apps link
Then click the Turn on option.
Now go back to the THUM program and click the Test Email button. It should display as shown below if successful.
Previous settings that used to work with Gmail - here for reference.