Fix POP mail parsing in multipart bounce e-mails.

This was originally authored by @stevesavanna in #707. This commit
contains changes and refactors that could not be pushed to the original PR.

Changes from #707

- Don't ignore bounce mails missing campaign / subscriber UUIDs. The
  original behaviour falls back to looking up subscribers by e-mail.
- Refactor repetetive header.get + regexp conditions per header into
  a simpler lookup map.
- Trim e-mail header values of `\r`.

Closes #707, #763

Co-authored-by: stevesavanna <steven@savannacorp.com>

internal/bounce/mailbox/pop.go

@@ -2,7 +2,9 @@ package mailbox
 
 import (
 	"encoding/json"
+	"io"
 	"regexp"
+	"strings"
 	"time"
 
 	"github.com/emersion/go-message"
@@ -17,9 +19,24 @@ type POP struct {
 	client *pop3.Client
 }
 
+type bounceHeaders struct {
+	Header string
+	Regexp *regexp.Regexp
+}
+
 var (
-	reCampUUID = regexp.MustCompile(`(?m)(?m:^` + models.EmailHeaderCampaignUUID + `:\s+?)([a-z0-9\-]{36})`)
-	reSubUUID  = regexp.MustCompile(`(?m)(?m:^` + models.EmailHeaderSubscriberUUID + `:\s+?)([a-z0-9\-]{36})`)
+	// List of header to look for in the e-mail body, regexp to fall back to if the header is empty.
+	headerLookups = []bounceHeaders{
+		{models.EmailHeaderCampaignUUID, regexp.MustCompile(`(?m)(?:^` + models.EmailHeaderCampaignUUID + `:\s+?)([a-z0-9\-]{36})`)},
+		{models.EmailHeaderSubscriberUUID, regexp.MustCompile(`(?m)(?:^` + models.EmailHeaderSubscriberUUID + `:\s+?)([a-z0-9\-]{36})`)},
+		{models.EmailHeaderDate, regexp.MustCompile(`(?m)(?:^` + models.EmailHeaderDate + `:\s+?)([\w,\,\ ,:,+,-]*(?:\(?:\w*\))?)`)},
+		{models.EmailHeaderFrom, regexp.MustCompile(`(?m)(?:^` + models.EmailHeaderFrom + `:\s+?)(.*)`)},
+		{models.EmailHeaderSubject, regexp.MustCompile(`(?m)(?:^` + models.EmailHeaderSubject + `:\s+?)(.*)`)},
+		{models.EmailHeaderMessageId, regexp.MustCompile(`(?m)(?:^` + models.EmailHeaderMessageId + `:\s+?)(.*)`)},
+		{models.EmailHeaderDeliveredTo, regexp.MustCompile(`(?m)(?:^` + models.EmailHeaderDeliveredTo + `:\s+?)(.*)`)},
+	}
+
+	reHdrReceived = regexp.MustCompile(`(?m)(?:^` + models.EmailHeaderReceived + `:\s+?)(.*)`)
 )
 
 // NewPOP returns a new instance of the POP mailbox client.
@@ -81,29 +98,51 @@ func (p *POP) Scan(limit int, ch chan models.Bounce) error {
 			return err
 		}
 
-		// Check if the identifiers are available in the parsed message.
-		var (
-			campUUID = m.Header.Get(models.EmailHeaderCampaignUUID)
-			subUUID  = m.Header.Get(models.EmailHeaderSubscriberUUID)
-		)
+		h := m
 
-		// If they are not, try to extract them from the message body.
-		if campUUID == "" {
-			if u := reCampUUID.FindSubmatch(b.Bytes()); len(u) == 2 {
-				campUUID = string(u[1])
-			}
-		}
-		if subUUID == "" {
-			if u := reSubUUID.FindSubmatch(b.Bytes()); len(u) == 2 {
-				subUUID = string(u[1])
+		// If this is a multipart message, find the last part.
+		if mr := m.MultipartReader(); mr != nil {
+			for {
+				part, err := mr.NextPart()
+				if err == io.EOF {
+					break
+				} else if err != nil {
+					return err
+				}
+				h = part
 			}
 		}
 
-		if campUUID == "" || subUUID == "" {
-			continue
+		// Reset the "unread portion" pointer of the message buffer.
+		// If you don't do this, you can't read the entire body because the pointer will not point to the beginning.
+		b, _ = c.RetrRaw(id)
+
+		// Lookup headers in the e-mail. If a header isn't found, fall back to regexp lookups.
+		hdr := make(map[string]string, 7)
+		for _, l := range headerLookups {
+			v := h.Header.Get(l.Header)
+
+			// Not in the header. Try regexp.
+			if v == "" {
+				if m := l.Regexp.FindAllSubmatch(b.Bytes(), -1); m != nil {
+					v = string(m[len(m)-1][1])
+				}
+			}
+
+			hdr[l.Header] = strings.TrimSpace(v)
 		}
 
-		date, _ := time.Parse("Mon, 02 Jan 2006 15:04:05 -0700", m.Header.Get("Date"))
+		// Received is a []string header.
+		msgReceived := h.Header.Map()[models.EmailHeaderReceived]
+		if len(msgReceived) == 0 {
+			if u := reHdrReceived.FindAllSubmatch(b.Bytes(), -1); u != nil {
+				for i := 0; i < len(u); i++ {
+					msgReceived = append(msgReceived, string(u[i][1]))
+				}
+			}
+		}
+
+		date, _ := time.Parse("Mon, 02 Jan 2006 15:04:05 -0700", hdr[models.EmailHeaderDate])
 		if date.IsZero() {
 			date = time.Now()
 		}
@@ -116,21 +155,21 @@ func (p *POP) Scan(limit int, ch chan models.Bounce) error {
 			DeliveredTo string   `json:"delivered_to"`
 			Received    []string `json:"received"`
 		}{
-			From:        m.Header.Get("From"),
-			Subject:     m.Header.Get("Subject"),
-			MessageID:   m.Header.Get("Message-Id"),
-			DeliveredTo: m.Header.Get("Delivered-To"),
-			Received:    m.Header.Map()["Received"],
+			From:        hdr[models.EmailHeaderFrom],
+			Subject:     hdr[models.EmailHeaderSubject],
+			MessageID:   hdr[models.EmailHeaderMessageId],
+			DeliveredTo: hdr[models.EmailHeaderDeliveredTo],
+			Received:    msgReceived,
 		})
 
 		select {
 		case ch <- models.Bounce{
 			Type:           "hard",
-			CampaignUUID:   campUUID,
-			SubscriberUUID: subUUID,
+			CampaignUUID:   hdr[models.EmailHeaderCampaignUUID],
+			SubscriberUUID: hdr[models.EmailHeaderSubscriberUUID],
 			Source:         p.opt.Host,
 			CreatedAt:      date,
-			Meta:           json.RawMessage(meta),
+			Meta:           meta,
 		}:
 		default:
 		}

models/models.go

@@ -69,6 +69,14 @@ const (
 	EmailHeaderSubscriberUUID = "X-Listmonk-Subscriber"
 	EmailHeaderCampaignUUID   = "X-Listmonk-Campaign"
 
+	// Standard e-mail headers.
+	EmailHeaderDate        = "Date"
+	EmailHeaderFrom        = "From"
+	EmailHeaderSubject     = "Subject"
+	EmailHeaderMessageId   = "Message-Id"
+	EmailHeaderDeliveredTo = "Delivered-To"
+	EmailHeaderReceived    = "Received"
+
 	BounceTypeHard = "hard"
 	BounceTypeSoft = "soft"
 )