From GMail mails are not sending, as we configure the mail alert from Gmail, sometimes it was not sending as it is due to Google as blocking the external smtp access for your account.
Route cause : The message could not sent to the smtp server may be you will receive this kind of response from the product and also you will receive an Email from Google on configured Email id. That Google Account: sign-in attempt blocked.
Solution : From received mail there will an link provided to enable the smtp access https://www.google.com/settings/security/lesssecureapps. Visit that link and enable the Turn On Option. Please refer the below image and follow the same. After that if you try to sent the mail from product it will works.