Don't use should validate_format_of(...) because that matcher works in weird ways. Use the allow_value matcher instead:

describe Email, '#sender' do
  # > Rspec 3 should syntax
  it { should allow_value("email@addresse.foo").for(:sender) }
  it { should_not allow_value("foo").for(:sender) }
  
  # Rspec 3 expect syntax
  it { is_expected.to allow_value("email@addresse.foo").for(:sender) }
  it { is_expected.not_to allow_value("foo").for(:sender) }
end

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Errors that may occur if you do use should validate_format_of(...):

NoMethodError: private method `gsub' called for /\A[a-z0-9\-_]+\z/:Regexp