queue: Internationalized Delivery Status Notifications (DSN)

Our non-delivery status notifications are quite simple today, but that
makes it much more difficult to support internationalization and
cross-language reporting.

There is a standard for internationalized DSNs, RFC 6533 (which builds
on top of the structured DSNs from RFC 3464).

This patch changes our DSN messages to be based on those standards, so
it is easier for MUAs to display reports according to the users'
languages preferences.

Note we still use message/rfc822 + 8bit to transmit the message, instead
of message/global, for compatibility reasons. This seems to be more
universally compatible, but the decision might be revisited in the
future. See RFC 5335 (section 4.6 in particular).

internal/queue/dsn_test.go

@@ -12,12 +12,12 @@ func TestDSN(t *testing.T) {
 		Message: Message{
 			ID:   <-newID,
 			From: "from@from.org",
-			To:   []string{"toto@africa.org", "negra@sosa.org"},
+			To:   []string{"ñaca@africa.org", "negra@sosa.org"},
 			Rcpt: []*Recipient{
 				{"poe@rcpt", Recipient_EMAIL, Recipient_FAILED,
-					"oh! horror!", "toto@africa.org"},
+					"oh! horror!", "ñaca@africa.org"},
 				{"newman@rcpt", Recipient_EMAIL, Recipient_PENDING,
-					"oh! the humanity!", "toto@africa.org"},
+					"oh! the humanity!", "ñaca@africa.org"},
 				{"ant@rcpt", Recipient_EMAIL, Recipient_SENT,
 					"", "negra@sosa.org"},
 			},
@@ -42,27 +42,60 @@ To: <from@from.org>
 Subject: Mail delivery failed: returning message to sender
 Message-ID: <chasquid-dsn-???????????@dsnDomain>
 Date: *
-X-Failed-Recipients: toto@africa.org, 
+In-Reply-To: *
+References: *
+X-Failed-Recipients: ñaca@africa.org, 
 Auto-Submitted: auto-replied
-
-Delivery to the following recipient(s) failed permanently:
-
-  - toto@africa.org
+MIME-Version: 1.0
+Content-Type: multipart/report; report-type=delivery-status;
+    boundary="???????????"
 
 
------ Technical details -----
+--???????????
+Content-Type: text/plain; charset="utf-8"
+Content-Disposition: inline
+Content-Description: Notification
+Content-Transfer-Encoding: 8bit
 
+Delivery of your message to the following recipient(s) failed permanently:
+
+  - ñaca@africa.org
+
+Technical details:
 - "poe@rcpt" (EMAIL) failed permanently with error:
     oh! horror!
-
 - "newman@rcpt" (EMAIL) failed repeatedly and timed out, last error:
     oh! the humanity!
 
 
------ Original message -----
+--???????????
+Content-Type: message/global-delivery-status
+Content-Description: Delivery Report
+Content-Transfer-Encoding: 8bit
+
+Reporting-MTA: dns; dsnDomain
+
+Original-Recipient: utf-8; ñaca@africa.org
+Final-Recipient: utf-8; poe@rcpt
+Action: failed
+Status: 5.0.0
+Diagnostic-Code: smtp; oh! horror!
+
+Original-Recipient: utf-8; ñaca@africa.org
+Final-Recipient: utf-8; newman@rcpt
+Action: failed
+Status: 4.0.0
+Diagnostic-Code: smtp; oh! the humanity!
+
+
+--???????????
+Content-Type: message/rfc822
+Content-Description: Undelivered Message
+Content-Transfer-Encoding: 8bit
 
 data ñaca
 
+--???????????--
 `
 
 // flexibleEq compares two strings, supporting wildcards.