If you are using PHP Mailer in a Simple Script For Send Email's Through Gmail, you might encounter "smtp error could not authenticate".
1. Log into your gmail account.
2. On the top find "My account" else goto "myaccount.google.com"
3. Under "Sign-in & security", Choose "Apps with account access".
4. Now turn ON "Allow less secure apps" (near the bottom of the page).
5. Now reload the page and try again.
It will work !!