Send email from Ruby on Rails (Action Mailer) over SMTP

# Action Mailer ships with Rails — plain Ruby: gem "net-smtp"

# config/environments/production.rb
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  address:              ENV.fetch("SMTP_HOST", "smtp.queensmtp.com"),
  port:                 ENV.fetch("SMTP_PORT", 587).to_i,
  user_name:            ENV.fetch("SMTP_USER"),
  password:             ENV.fetch("SMTP_PASS"),
  authentication:       :plain,
  enable_starttls:      true,      # 587; for 465 use tls: true instead
  open_timeout:         20,
  read_timeout:         20,
}
config.action_mailer.default_url_options = { host: "yourdomain.com" }
config.action_mailer.raise_delivery_errors = true

# app/mailers/order_mailer.rb
class OrderMailer < ApplicationMailer
  default from: "Your App <hello@yourdomain.com>"

  def shipped(order)
    @order = order
    mail(to: order.customer_email, subject: "Your order has shipped")
  end
end

# anywhere
OrderMailer.shipped(order).deliver_later    # via Active Job; deliver_now to send inline

# plain Ruby (no Rails)
require "net/smtp"
msg = "From: hello@yourdomain.com\nTo: customer@example.com\nSubject: test\n\nSMTP works\n"
Net::SMTP.start("smtp.queensmtp.com", 587, "localhost", ENV["SMTP_USER"], ENV["SMTP_PASS"], :plain, tls: false, starttls: :always) do |smtp|
  smtp.send_message msg, "hello@yourdomain.com", "customer@example.com"
end