Net::SMTPAuthenticationError (535-5.7.8 Username and Password not accepted

config.action_mailer.default_url_options = { :host => 'localhost'} 
  ActionMailer::Base.smtp_settings = {
    :user_name => 'xyz@gmail.com',
    :password => 'nico6655',
    :domain => 'gmail.com',
    :address => 'smtp.gmail.com',
    :port => 587,
    :authentication => :plain, 
    :enable_starttls_auto => true,
    :openssl_verify_mode  => 'none'

  }

How to fix the 'SMTP could not authenticate' error?