diff --git a/lib/prism/mailer/message.rb b/lib/prism/mailer/message.rb
new file mode 100644
index 0000000..277a96a
--- /dev/null
+++ b/lib/prism/mailer/message.rb
@@ -0,0 +1,131 @@
+require 'securerandom'
+require 'time'
+
+module Prism
+  module Mailer
+    # A single outgoing email: headers, a text and/or HTML body, and
+    # attachments. #to_s renders a real RFC 5322 message (multipart/mixed
+    # wrapping multipart/alternative when both a body variant and
+    # attachments are present) that a real SMTP server or Net::SMTP client
+    # can transmit as-is.
+    class Message
+      attr_accessor :from, :reply_to, :subject, :text_body, :html_body
+      attr_reader :to, :cc, :bcc, :attachments, :headers
+
+      def initialize
+        @to = []
+        @cc = []
+        @bcc = []
+        @attachments = []
+        @headers = {}
+        @message_id = "#{SecureRandom.hex(16)}@prism"
+      end
+
+      def to=(value)
+        @to = Array(value)
+      end
+
+      def cc=(value)
+        @cc = Array(value)
+      end
+
+      def bcc=(value)
+        @bcc = Array(value)
+      end
+
+      def []=(header, value)
+        headers[header.to_s] = value
+      end
+
+      def add_attachment(filename, content, mime_type: 'application/octet-stream')
+        attachments << { filename: filename, content: content, mime_type: mime_type }
+      end
+
+      def all_recipients
+        (to + cc + bcc).compact
+      end
+
+      def multipart_body?
+        !text_body.nil? && !html_body.nil?
+      end
+
+      def to_s
+        raise ArgumentError, 'message requires a from address' unless from
+        raise ArgumentError, 'message requires at least one recipient (to/cc/bcc)' if all_recipients.empty?
+        raise ArgumentError, 'message requires a text_body or html_body' if text_body.nil? && html_body.nil?
+
+        lines = header_lines
+        body = build_body
+
+        lines.concat(body[:headers])
+        lines << ''
+        lines << body[:content]
+        lines.join("\r\n")
+      end
+
+      private
+
+      def header_lines
+        lines = []
+        lines << "From: #{from}"
+        lines << "To: #{to.join(', ')}" unless to.empty?
+        lines << "Cc: #{cc.join(', ')}" unless cc.empty?
+        lines << "Reply-To: #{reply_to}" if reply_to
+        lines << "Subject: #{encode_header(subject.to_s)}"
+        lines << "Date: #{Time.now.httpdate}"
+        lines << "Message-ID: <#{@message_id}>"
+        lines << 'MIME-Version: 1.0'
+        headers.each { |name, value| lines << "#{name}: #{value}" }
+        lines
+      end
+
+      def build_body
+        alternative = build_alternative_part
+        return alternative if attachments.empty?
+
+        boundary = new_boundary('mixed')
+        content = +"--#{boundary}\r\n"
+        content << alternative[:headers].join("\r\n")
+        content << "\r\n\r\n"
+        content << alternative[:content]
+        attachments.each { |attachment| content << "\r\n--#{boundary}\r\n#{attachment_part(attachment)}" }
+        content << "\r\n--#{boundary}--"
+        { headers: ["Content-Type: multipart/mixed; boundary=\"#{boundary}\""], content: content }
+      end
+
+      def build_alternative_part
+        if multipart_body?
+          boundary = new_boundary('alt')
+          content = +"--#{boundary}\r\n"
+          content << "Content-Type: text/plain; charset=UTF-8\r\n\r\n#{text_body}"
+          content << "\r\n--#{boundary}\r\n"
+          content << "Content-Type: text/html; charset=UTF-8\r\n\r\n#{html_body}"
+          content << "\r\n--#{boundary}--"
+          { headers: ["Content-Type: multipart/alternative; boundary=\"#{boundary}\""], content: content }
+        elsif html_body
+          { headers: ['Content-Type: text/html; charset=UTF-8'], content: html_body }
+        else
+          { headers: ['Content-Type: text/plain; charset=UTF-8'], content: text_body }
+        end
+      end
+
+      def attachment_part(attachment)
+        header = +"Content-Type: #{attachment[:mime_type]}; name=\"#{attachment[:filename]}\"\r\n"
+        header << "Content-Transfer-Encoding: base64\r\n"
+        header << "Content-Disposition: attachment; filename=\"#{attachment[:filename]}\"\r\n\r\n"
+        header << [attachment[:content]].pack('m0').scan(/.{1,76}/).join("\r\n")
+        header
+      end
+
+      def new_boundary(label)
+        "prism-#{label}-#{SecureRandom.hex(8)}"
+      end
+
+      def encode_header(text)
+        return text if text.ascii_only?
+
+        "=?UTF-8?B?#{[text].pack('m0')}?="
+      end
+    end
+  end
+end
diff --git a/lib/prism/mailer/preview.rb b/lib/prism/mailer/preview.rb
new file mode 100644
index 0000000..fd74543
--- /dev/null
+++ b/lib/prism/mailer/preview.rb
@@ -0,0 +1,44 @@
+module Prism
+  module Mailer
+    # ActionMailer::Preview-lite: subclass and define one instance method per
+    # preview action that returns a mailer Delivery built from fixture data
+    # (`UserMailer.welcome(sample_user)`) instead of a real request/record -
+    # so an email's rendered HTML/text can be inspected without sending it or
+    # ever touching Mailer.deliver!. Every subclass self-registers so a host
+    # app can list and browse them (e.g. mount them under a dev-only route).
+    class Preview
+      class << self
+        def registry
+          @registry ||= []
+        end
+
+        def inherited(subclass)
+          registry << subclass
+        end
+
+        def preview_name
+          Inflector.underscore(name.to_s.split('::').last.sub(/Preview\z/, ''))
+        end
+
+        def emails
+          instance_methods(false).map(&:to_s).sort
+        end
+
+        # Renders one preview action by name and returns its Delivery
+        # (message.html_body / message.text_body / message.subject, ...);
+        # never calls #deliver_now or #deliver_later.
+        def call(email)
+          email = email.to_s
+          unless emails.include?(email)
+            raise ArgumentError, "unknown preview #{email.inspect} for #{self} (known: #{emails.join(', ')})"
+          end
+          new.public_send(email)
+        end
+
+        def find(preview_name)
+          registry.find { |klass| klass.preview_name == preview_name.to_s }
+        end
+      end
+    end
+  end
+end