Action mailer - Sending an email through a contact form with Ruby on Rails

I'm using Ruby 2.6.3 and Rails 5.2. I want to make a contact form on my website, with a box for user email address, and a box for the message content. When a user fill those boxes and click on "Submit", i would like that it send me an email. No need for the user to sign up or anything.

My website is a one page website, i don't have any models, and i just have 1 controller : pages_controller.rb.

I used the documentation on Action mailer but i didn't suceed.

Here are the steps i followed :

rails generate mailer ContactMailer

config/environments/development.rb :

config.action_mailer.default charset: 'utf-8'
config.action_mailer.perform_deliveries = true
config.action_mailer.default_url_options = { :host => 'localhost:3000' }
config.action_mailer.raise_delivery_errors = true    
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
    address: "smtp.mail.yahoo.com",
    port: 465,
    domain: "yahoo.com",
    authentication: "plain",
    enable_starttls_auto: true,
    user_name: "myemailaddress@yahoo.com",
    password: "********"
  }

app/mailers/contact_mailer.rb :

class ContactMailer < ApplicationMailer
  default from: "from@example.com"
  def contact(message)
    @message = message
    mail(to: 'myemailaddress@yahoo.com', subject: 'Test')
  end
end

app/views/contact_mailer/contact.html.erb :

<p>Hello world</p>
<%= @message %>

app/controllers/pages_controller.rb :

class PagesController < ApplicationController
  def home
  end
  def send_contact
     ContactMailer.contact(params[:message]).deliver
  end
end

app/views/pages/home.html.erb :

<form action="/send_contact" method="post">
   <input name="email" type="email" class="form-control mb-1" placeholder="Your email">
   <textarea name="message" class="form-control" placeholder="Your message"></textarea>
   <button type="submit">Submit</button>
</form>

config/routes.rb :

Rails.application.routes.draw do
  root to: 'pages#home'
  post "send_contact" => "pages#send_contact"
end

This is supposed to send me an email when clicking on submit. Instead, i get this error : EOFError in PagesController#send_contact end of file

So i tried with a gmail address with the right username / password and the right config :

config.action_mailer.smtp_settings = {
    address: "smtp.gmail.com",
    port: 587,
    domain: "gmail.com",
    authentication: "plain",
    user_name: ENV["USERNAME"],
    password: ENV["PASSWORD"]
  }

And i have this error : 535-5.7.8 Username and Password not accepted.. I tried to allow "less secure apps" and desactivate 2FA. I also tried to generate an app specific password. Same error.

I don't understand what is going on.

EDIT :

When i printed ENV["USERNAME"] i didn't have what i wrote in my .env file. I had instead my first name, i don't know why. So i changed my .env file to MAIL_USERNAME (instead of USERNAME) and now it's working in development, but unfortunately i get an error 500 in production, and i have no idea how to debug this.

EDIT 2 :

Problem solved ! I just had to add my .env variables in heroku