Rails Custom Validator for email with I18n message. Regex is simple but should be enough. Add https://github.com/mailcheck/mailcheck for even less invalid emails :-)

application.rb

# config/application.rb
[...]
    config.autoload_paths += %W["#{config.root}/app/validators/"]
[...]

email_validator.rb

# app/validators/email_validator.rb
class EmailValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value =~ /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i
      record.errors.add(attribute, (options[:message] || :email_format))
    end
  end
end

fr.yml

# config/locales/fr.yml
fr:
  activerecord:
    errors:
      email_format: "doit être une adresse valide"

my_model.rb

# app/models/my_model.rb
[...]
  validates :email_attribute, email: true
[...]