mailer with gmail gives Net::SMTPAuthenticationError: with 2-factor auth disabled

I am getting a Net::SMTPAuthenticationError both on my development (local) env as well as when I host on Heroku, when I try to send mails from my rails application.

This started after sending the first couple of email (2 mails to be specific).

All research so far made me think that this should could be due to having a 2-factor auth enabled on the gmail account, but when I checked the account, 2-factor autt was not enabled.

below is my smtp setting both in config/development.rb and config/production.rb:

  config.action_mailer.delivery_method = :smtp
  # SMTP settings for gmail
  config.action_mailer.smtp_settings = {
      :address              => "smtp.gmail.com",
      :port                 => 587,
      :user_name            => Rails.configuration.gmail_address,
      :password             => Rails.configuration.gmail_password,
      :authentication       => "plain",
      :enable_starttls_auto => true
  }

My mailer is set up as follow:

class StackoMailer < ActionMailer::Base

    def success_mail
      @my_email = Rails.configuration.gmail_address
      mail(
        from: 'notifications@StackoProbe.com',
        to: @my_email,
        subject: 'Fuck Yeah! Visited successfully...'
      )
    end
end

On calling PushWatir::StackoMailer.success_mail.deliver_now!, I get the following error:

2.2.1 :001 > PushWatir::StackoMailer.success_mail.deliver_now!
Net::SMTPAuthenticationError: 534-5.7.14 <http://ift.tt/1NOZYnQ

    from /Users/andela/.rvm/rubies/ruby-2.2.1/lib/ruby/2.2.0/net/smtp.rb:976:in `check_auth_response'
    from /Users/andela/.rvm/rubies/ruby-2.2.1/lib/ruby/2.2.0/net/smtp.rb:740:in `auth_plain'
    from /Users/andela/.rvm/rubies/ruby-2.2.1/lib/ruby/2.2.0/net/smtp.rb:732:in `authenticate'
    from /Users/andela/.rvm/rubies/ruby-2.2.1/lib/ruby/2.2.0/net/smtp.rb:567:in `do_start'
    from /Users/andela/.rvm/rubies/ruby-2.2.1/lib/ruby/2.2.0/net/smtp.rb:520:in `start'
    from /Users/andela/.rvm/gems/ruby-2.2.1/gems/mail-2.6.3/lib/mail/network/delivery_methods/smtp.rb:112:in `deliver!'
    from /Users/andela/.rvm/gems/ruby-2.2.1/gems/mail-2.6.3/lib/mail/message.rb:252:in `deliver!'
    from /Users/andela/.rvm/gems/ruby-2.2.1/gems/actionmailer-4.2.4/lib/action_mailer/message_delivery.rb:77:in `deliver_now!'
    from (irb):1
    from /Users/andela/.rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands/console.rb:110:in `start'
    from /Users/andela/.rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands/console.rb:9:in `start'
    from /Users/andela/.rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands/commands_tasks.rb:68:in `console'
    from /Users/andela/.rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
    from /Users/andela/.rvm/gems/ruby-2.2.1/gems/railties-4.2.4/lib/rails/commands.rb:17:in `<top (required)>'
    from bin/rails:8:in `require'
    from bin/rails:8:in `<main>'
2.2.1 :002 > 

What could be causing this?

How can I make this work? Big thanks for responses