Feat: Add message token to return path

[jmdunsing]

This PR incorporates a minor change to the way the return path is generated to allow for additional bounce processing and message forwarding. Those using postal strictly as a transactional email solution should notice no difference in performance. Those who wish to be able to capture all hard bounces and those who use postal as a marketing solution will have another method besides the X-Postal-MsgID header to identify bounces and replies, since the message token will be included in the return path.

Background:
Some mail servers return a bounce but omit identifiable information and strip the X-Postal-MsgID header from the email, leaving postal users unable to prevent future hard bounces. This change will provide the ability for a subsequent coding solution to tie bounces and replies to emails to their original sender.

This design change is consistent with the current bounce design, which splits the domain path on "@" then subsequently on "+", and checks the uname to see if the server was a bounce (specifically line 328), see

postal/app/lib/smtp_server/client.rb

Lines 303 to 340 in 6df9636

	def rcpt_to(data)
	unless in_state(:mail_from_received, :rcpt_to_received)
	increment_error_count("rcpt-to-out-of-order")
	return "503 EHLO/HELO and MAIL FROM first please"
	end
	rcpt_to = data.gsub(/RCPT TO\s*:\s*/i, "").gsub(/.*</, "").gsub(/>.*/, "").strip
	if rcpt_to.blank?
	increment_error_count("empty-rcpt-to")
	return "501 RCPT TO should not be empty"
	end
	uname, domain = rcpt_to.split("@", 2)
	if domain.blank?
	increment_error_count("invalid-rcpt-to")
	return "501 Invalid RCPT TO"
	end
	uname, tag = uname.split("+", 2)
	if domain == Postal::Config.dns.return_path_domain || domain =~ /\A#{Regexp.escape(Postal::Config.dns.custom_return_path_prefix)}\./
	# This is a return path
	@state = :rcpt_to_received
	if server = ::Server.where(token: uname).first
	if server.suspended?
	increment_error_count("server-suspended")
	"535 Mail server has been suspended"
	else
	logger&.debug "Added bounce on server #{server.id}"
	@recipients << [:bounce, rcpt_to, server]
	"250 OK"
	end
	else
	increment_error_count("invalid-server-token")
	"550 Invalid server token"
	end

:

These bounces will still not be identified by postal as bounces, and the incoming messages will still hardfail, because the current code still searches exclusively for the X-Postal-MsgID header

postal/lib/postal/message_db/message.rb

Lines 496 to 508 in 6df9636

	#
	# Return a message object that this message is a reply to
	#
	def original_messages
	return nil unless bounce
	other_message_ids = raw_message.scan(/\X-Postal-MsgID:\s*([a-z0-9]+)/i).flatten
	if other_message_ids.empty?
	[]
	else
	database.messages(where: { token: other_message_ids })
	end
	end

:

[jmdunsing]

I created an image in my fork of postal and emails are being sent correctly using the new method.

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[ghost]

i like this

[jmdunsing]

What needs to be done to get this change incorporated?

[schueffi]

In our current setup (not using postal, but "exim"), we are using the exact same approach to have a "dedicated" bounce address per sent message. This way, we unambiguously can match incoming bounces to the corresponding outgoing emails without relying on still-included headers or message-ids.
The commit in this pull request just fulfills the "first half" of the process: adding individual bounce addresses. The "recognition" of bounces still relies on the X-Postal-MsgID header. This even could be supplemented by an additional search by the "tag" (as given in the new bounce-address after the "+" sign) - then, no "special" / "custom" processing of all "unmatched" bounces would be required. Optionally, this additional tag-based mail search could be enabled by some environmental / config setting.

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[github-actions]

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