widgets/compose: Generate Message ID with domain name from address

go-messages GenerateMessageID does generate the Message ID based on the
hostname of the local machine. This is ok in this context because the
lib does not know really anything of the surroundings. But aerc does
know about the sender domain and could use this instead of the hostname.
This also aligns with the recommendation of RFC4322 to use a unique
domain name, to ensure the uniqueness of the whole Message ID [1].

A side effect of this change is that it also improves the score of some
spam filters a bit, if an E-Mail has a fully qualified domain name after
the @.

This patch fixes https://todo.sr.ht/~sircmpwn/aerc2/526

[1] https://www.rfc-editor.org/rfc/rfc5322.html#section-3.6.4

1 files changed, 25 insertions, 3 deletions

diff --git a/widgets/compose.go b/widgets/compose.go
index 01c6281..91599c6 100644
--- a/widgets/compose.go
+++ b/widgets/compose.go
@@ -3,15 +3,18 @@ package widgets
 import (
 	"bufio"
 	"bytes"
+	"encoding/binary"
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math/rand"
 	"mime"
 	"net/http"
 	"net/textproto"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -381,9 +384,24 @@ func (c *Composer) PrepareHeader() (*mail.Header, error) {
 	// control headers not normally set by the user
 	// repeated calls to PrepareHeader should be a noop
 	if !c.header.Has("Message-Id") {
-		err := c.header.GenerateMessageID()
-		if err != nil {
-			return nil, err
+		if c.header.Has("From") {
+			// generate the Message-Id our self, to include a FQDN, code is based on the GenerateMessageID of go-message
+			now := uint64(time.Now().UnixNano())
+
+			nonceByte := make([]byte, 8)
+			if _, err := rand.Read(nonceByte); err != nil {
+				return nil, err
+			}
+			nonce := binary.BigEndian.Uint64(nonceByte)
+
+			fromAddresses, err := c.header.AddressList("From")
+			if err != nil {
+				return nil, err
+			}
+			domain := strings.Split(fromAddresses[0].Address, "@")[1]
+
+			msgID := fmt.Sprintf("%s.%s@%s", base36(now), base36(nonce), domain)
+			c.header.Set("Message-Id", "<"+msgID+">")
 		}
 	}
 	if !c.header.Has("Date") {
@@ -392,6 +410,10 @@ func (c *Composer) PrepareHeader() (*mail.Header, error) {
 	return c.header, nil
 }
 
+func base36(input uint64) string {
+	return strings.ToUpper(strconv.FormatUint(input, 36))
+}
+
 func (c *Composer) WriteMessage(header *mail.Header, writer io.Writer) error {
 	if err := c.reloadEmail(); err != nil {
 		return err