535-5.7.8 Username and Password not accepted

I had the same problem. Now its working fine after doing below changes.

https://www.google.com/settings/security/lesssecureapps

You should change the "Access for less secure apps" to Enabled (it was enabled, I changed to disabled and than back to enabled). After a while I could send email.

First, You need to use a valid Gmail account with your credentials.

Second, In my app I don't use TLS auto, try without this line:

config.action_mailer.smtp_settings = {
  address:              'smtp.gmail.com',
  port:                 587,
  domain:               'gmail.com',
  user_name:            '[email protected]',
  password:             'YOUR_PASSWORD',
  authentication:       'plain'
  # enable_starttls_auto: true
  # ^ ^ remove this option ^ ^
}

UPDATE: (See answer below for details) now you need to enable "less secure apps" on your Google Account

https://myaccount.google.com/lesssecureapps?pli=1

Time flies, the way I do without enabling less secured app is making a password for specific app

Step one: enable 2FA

• https://myaccount.google.com/signinoptions/two-step-verification/enroll-welcome

Step two: create an app-specific password

• https://myaccount.google.com/apppasswords

After this, put the sixteen digits password to the settings and reload the app, enjoy!

  config.action_mailer.smtp_settings = {
    ...
    password: 'HERE', # <---
    authentication: 'plain',
    enable_starttls_auto: true
  }