In most cases nowadays, users enable 2-factor authentication in Gmail settings. This leads to impossibility to enable less-secure apps, and thus sending emails from TBM using Gmail accounts. To correctly configure Gmail account, please go to the following page: https://myaccount.google.com/u/1/apppasswords, and create a new password there for mail. Then copy generated password, and use it on the e-mail configuration page. For reference, please check the following screenshot: