diff --git a/test/test_mailer_message.rb b/test/test_mailer_message.rb
new file mode 100644
index 0000000..a790f9c
--- /dev/null
+++ b/test/test_mailer_message.rb
@@ -0,0 +1,99 @@
+require 'minitest/autorun'
+require 'prism'
+
+class MailerMessageTest < Minitest::Test
+  def setup
+    @message = Prism::Mailer::Message.new
+    @message.from = 'sender@example.com'
+    @message.to = 'receiver@example.com'
+    @message.subject = 'Test subject'
+  end
+
+  def test_requires_a_from_address
+    @message.from = nil
+    @message.text_body = 'body'
+    assert_raises(ArgumentError) { @message.to_s }
+  end
+
+  def test_requires_at_least_one_recipient
+    @message.to = []
+    @message.text_body = 'body'
+    assert_raises(ArgumentError) { @message.to_s }
+  end
+
+  def test_requires_a_body
+    assert_raises(ArgumentError) { @message.to_s }
+  end
+
+  def test_to_cc_bcc_coerce_a_single_address_into_an_array
+    @message.to = 'a@example.com'
+    @message.cc = 'b@example.com'
+    @message.bcc = 'c@example.com'
+    assert_equal ['a@example.com'], @message.to
+    assert_equal %w[a@example.com b@example.com c@example.com], @message.all_recipients
+  end
+
+  def test_plain_text_only_message_has_a_single_text_content_type
+    @message.text_body = 'hello world'
+    raw = @message.to_s
+    assert_includes raw, "Content-Type: text/plain; charset=UTF-8\r\n\r\nhello world"
+    refute_includes raw, 'multipart'
+  end
+
+  def test_html_only_message_has_a_single_html_content_type
+    @message.html_body = '<p>hello</p>'
+    raw = @message.to_s
+    assert_includes raw, "Content-Type: text/html; charset=UTF-8\r\n\r\n<p>hello</p>"
+    refute_includes raw, 'multipart'
+  end
+
+  def test_text_and_html_together_build_a_multipart_alternative_part
+    @message.text_body = 'plain body'
+    @message.html_body = '<p>html body</p>'
+    raw = @message.to_s
+    assert_match(/Content-Type: multipart\/alternative; boundary="([^"]+)"/, raw)
+    boundary = raw[/boundary="([^"]+)"/, 1]
+    assert_includes raw, "--#{boundary}\r\nContent-Type: text/plain"
+    assert_includes raw, "--#{boundary}\r\nContent-Type: text/html"
+    assert_includes raw, "--#{boundary}--"
+  end
+
+  def test_attachment_wraps_the_body_in_multipart_mixed_and_base64_encodes_content
+    @message.text_body = 'see attached'
+    @message.add_attachment('data.bin', "\xFF\x00\xFE binary".b, mime_type: 'application/octet-stream')
+    raw = @message.to_s
+    assert_match(/Content-Type: multipart\/mixed; boundary="([^"]+)"/, raw)
+    assert_includes raw, 'Content-Disposition: attachment; filename="data.bin"'
+    assert_includes raw, 'Content-Transfer-Encoding: base64'
+    encoded = raw[/Content-Disposition: attachment; filename="data\.bin"\r\n\r\n(.+?)\r\n--/m, 1]
+    assert_equal "\xFF\x00\xFE binary".b, encoded.unpack1('m0')
+  end
+
+  def test_headers_writer_adds_a_custom_header_line
+    @message.text_body = 'body'
+    @message['X-Custom'] = 'yes'
+    assert_includes @message.to_s, "X-Custom: yes\r\n"
+  end
+
+  def test_reply_to_and_cc_appear_in_headers
+    @message.text_body = 'body'
+    @message.cc = 'cc@example.com'
+    @message.reply_to = 'reply@example.com'
+    raw = @message.to_s
+    assert_includes raw, "Cc: cc@example.com\r\n"
+    assert_includes raw, "Reply-To: reply@example.com\r\n"
+  end
+
+  def test_non_ascii_subject_is_mime_encoded
+    @message.subject = "Zaproszenie na przyj\u0119cie"
+    @message.text_body = 'body'
+    subject_line = @message.to_s.lines.find { |l| l.start_with?('Subject:') }
+    assert_match(/Subject: =\?UTF-8\?B\?.+\?=/, subject_line)
+  end
+
+  def test_ascii_subject_is_left_untouched
+    @message.text_body = 'body'
+    subject_line = @message.to_s.lines.find { |l| l.start_with?('Subject:') }
+    assert_equal 'Subject: Test subject', subject_line.chomp("\n")
+  end
+end
diff --git a/test/test_mailer_preview.rb b/test/test_mailer_preview.rb
new file mode 100644
index 0000000..7c37667
--- /dev/null
+++ b/test/test_mailer_preview.rb
@@ -0,0 +1,70 @@
+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+require 'fileutils'
+
+class MailerPreviewTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-mailer-preview')
+    FileUtils.mkdir_p(File.join(@dir, 'preview_user'))
+    File.write(File.join(@dir, 'preview_user/welcome.text.erb'), "Hi <%= name %>\n")
+
+    mailer_dir = @dir
+    @mailer_class = Class.new(Prism::Mailer::Base) do
+      self.template_roots = [mailer_dir]
+
+      def welcome(name)
+        @name = name
+        mail(to: 'someone@example.com', subject: 'Welcome!')
+      end
+    end
+    Object.const_set(:PreviewUserMailer, @mailer_class)
+
+    mailer_class = @mailer_class
+    @preview_class = Class.new(Prism::Mailer::Preview) do
+      define_method(:welcome) { mailer_class.welcome('Fixture Fred') }
+    end
+    Object.const_set(:UserMailerPreview, @preview_class)
+  end
+
+  def teardown
+    Prism::Mailer::Preview.registry.delete(@preview_class)
+    %i[UserMailerPreview PreviewUserMailer].each do |name|
+      Object.send(:remove_const, name) if Object.const_defined?(name)
+    end
+    FileUtils.remove_entry(@dir)
+  end
+
+  def test_subclassing_registers_the_preview
+    assert_includes Prism::Mailer::Preview.registry, UserMailerPreview
+  end
+
+  def test_preview_name_strips_the_preview_suffix_and_underscores
+    assert_equal 'user_mailer', UserMailerPreview.preview_name
+  end
+
+  def test_emails_lists_the_defined_preview_actions
+    assert_equal ['welcome'], UserMailerPreview.emails
+  end
+
+  def test_call_renders_the_action_and_returns_its_delivery
+    delivery = UserMailerPreview.call(:welcome)
+    assert_equal "Hi Fixture Fred\n", delivery.message.text_body
+  end
+
+  def test_call_never_delivers_the_message
+    Prism::Mailer.deliveries.clear
+    UserMailerPreview.call(:welcome)
+    assert_empty Prism::Mailer.deliveries
+  end
+
+  def test_call_raises_for_an_unknown_email
+    error = assert_raises(ArgumentError) { UserMailerPreview.call(:does_not_exist) }
+    assert_match(/unknown preview/, error.message)
+  end
+
+  def test_find_looks_up_a_registered_preview_by_name
+    assert_equal UserMailerPreview, Prism::Mailer::Preview.find('user_mailer')
+    assert_nil Prism::Mailer::Preview.find('nonexistent')
+  end
+end