```ruby
SMTP_SETTINGS = {
  address: ENV.fetch("SMTP_ADDRESS"), # example: "smtp.sendgrid.net"
  authentication: :plain,
  domain: ENV.fetch("SMTP_DOMAIN"), # example: "heroku.com"
  enable_starttls_auto: true,
  password: ENV.fetch("SMTP_PASSWORD"),
  port: "587",
  user_name: ENV.fetch("SMTP_USERNAME")
}
```

Add option to use Mailgun addon keys.
user
```ruby
SMTP_SETTINGS = {
  address: ENV["SMTP_ADDRESS"] || ENV["MAILGUN_SMTP_SERVER"], # example: "smtp.sendgrid.net"
  authentication: :plain,
  domain: ENV["SMTP_DOMAIN"] || ENV["MAILGUN_DOMAIN"], # example: "heroku.com"
  enable_starttls_auto: true,
  password: ENV["SMTP_PASSWORD"] || ENV["MAILGUN_SMTP_PASSWORD"],
  port: "587",
  user_name: ENV["SMTP_USERNAME"] || ENV["MAILGUN_SMTP_LOGIN"]
}
```
assistant