=== Buggy ===

class EmailAddress
  include Comparable

  def initialize(string)
    if string =~ /@/
      @raw_email_address = string.downcase.strip
    else
      raise ArgumentError, "email address must have an '@'"
    end
  end

  def <=>(other)
    raw_email_address <=> other.to_s # <-- BUG IS HERE
  end

  def to_s
    raw_email_address
  end

  protected

  attr_reader :raw_email_address
end


=== Test ===

irb(main):024:0> EmailAddress.new("user@example.com") == EmailAddress.new("user@example.com")
=> true
irb(main):025:0> EmailAddress.new("user@example.com") == EmailAddress.new("User@example.com")
=> true
irb(main):026:0> EmailAddress.new("user@example.com") == "user@example.com"
=> true
irb(main):027:0> EmailAddress.new("user@example.com") == "User@example.com"
=> false

=== Fixed ===

class EmailAddress
  include Comparable

  def initialize(string)
    if string =~ /@/
      @raw_email_address = string.downcase.strip
    else
      raise ArgumentError, "email address must have an '@'"
    end
  end

  def <=>(other)
    raw_email_address <=> other.to_s.downcase.strip # <<-- FIXED HERE
  end

  def to_s
    raw_email_address
  end

  protected

  attr_reader :raw_email_address
end

=== Test ===

irb(main):024:0> EmailAddress.new("user@example.com") == EmailAddress.new("user@example.com")
=> true
irb(main):025:0> EmailAddress.new("user@example.com") == EmailAddress.new("User@example.com")
=> true
irb(main):026:0> EmailAddress.new("user@example.com") == "user@example.com"
=> true
irb(main):027:0> EmailAddress.new("user@example.com") == "User@example.com"
=> true