I was also facing the issue like 'The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.0 Authentication Required' then went through so much internet materials but it didn't helped me fully. How I solved it like
step1:smtp.gmail.com is gmail server so go to your account gmail settings->click on see all settings->Forwarding and IMAP/POP->check pop and imap is enabled ,if not enable it->Save changes. step2-click on your gmail profile picture->click on Manage your google account->go to security tab->check for Access to less secure apps(this option will be available if you havent opt for two step verification)->by default google will set it as disable, make it enable to use your real gmail password working for sending email. note:-Enabling gmail access for less secure apps,may be dangerous for you so i dont recommend this
step3:-if your account has two step verification enabled or want to use password other than your gmail Real password using app specific password then try this:- click on your gmail profile picture->click on Manage your google account->go to security tab->search for APP PASSWORD->select any app name given->select any device name->click on generate->copy the 16-digit password and paste it into your app where you have to enter a gmail password in place of your real gmail password.