fix: generate Message-ID once and persist to DB (#328)

Previously, a random suffix was generated every time BuildBytes or
ForwardBuildBytes was called, causing IMAP/POP3 clients to treat the
same message as a new message on every sync and breaking conversation
threading.

Changes:
- Add msg_id column to the email table (synced automatically via Sync2)
- Generate a crypto/rand-based RFC-compliant Message-ID once at email
  creation time (send.go for outgoing, read_content.go for incoming)
- Preserve the original Message-ID from externally received emails
- Load the persisted MsgID when rebuilding Email from DB records
- Use the stored MsgID in BuildBytes, ForwardBuildBytes and the IMAP
  ENVELOPE response; fall back to the old numeric format for legacy rows

Fixes the MSGID_SHORT SpamAssassin penalty and Gmail rejection while
keeping Message-IDs stable across syncs.

Co-authored-by: root <root@Claw-ai.tail1478c.ts.net>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

server/controllers/email/send.go

@@ -191,6 +191,7 @@ func Send(ctx *context.Context, w http.ResponseWriter, req *http.Request) {
 		CronSendTime: time.Now(),
 		Status:       1,
 		CreateTime:   time.Now(),
+		MsgID:        parsemail.GenerateMsgID(config.Instance.Domain),
 	}
 
 	_, err = db.Instance.Insert(&modelEmail)
@@ -202,6 +203,7 @@ func Send(ctx *context.Context, w http.ResponseWriter, req *http.Request) {
 	}
 
 	e.MessageId = cast.ToInt64(modelEmail.Id)
+	e.MsgID = modelEmail.MsgID
 
 	async.New(ctx).Process(func(p any) {
 		errMsg := ""

server/dto/parsemail/email.go

@@ -2,6 +2,7 @@ package parsemail
 
 import (
 	"bytes"
+	"crypto/rand"
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
@@ -68,9 +69,18 @@ type Email struct {
 	Date        string
 	Status      int // 0未发送，1已发送，2发送失败，3删除，5广告邮件
 	MessageId   int64
+	MsgID       string // RFC-compliant Message-ID, persisted in DB
 	Size        int
 }
 
+// GenerateMsgID creates an RFC-compliant Message-ID unique enough to avoid spam filters.
+// It must be called once at email creation time and the result persisted to DB.
+func GenerateMsgID(domain string) string {
+	randBytes := make([]byte, 16)
+	_, _ = rand.Read(randBytes)
+	return fmt.Sprintf("%x.%d@%s", randBytes, time.Now().UnixNano(), domain)
+}
+
 // Xss filter policy
 var (
 	strictPolicy  *bluemonday.Policy
@@ -193,6 +203,7 @@ func NewEmailFromModel(d models.Email) *Email {
 
 	return &Email{
 		MessageId: cast.ToInt64(d.Id),
+		MsgID:     d.MsgID,
 		From: &User{
 			Name:         d.FromName,
 			EmailAddress: d.FromAddress,
@@ -218,6 +229,10 @@ func NewEmailFromReader(to []string, r io.Reader, size int) *Email {
 	}
 
 	ret.Size = size
+	// Preserve the original Message-ID from the sender so it is stored and reused consistently.
+	if mid := m.Header.Get("Message-Id"); mid != "" {
+		ret.MsgID = strings.TrimPrefix(strings.TrimSuffix(strings.TrimSpace(mid), ">"), "<")
+	}
 	ret.From = buildUser(m.Header.Get("From"))
 
 	smtpTo := buildUsers(to)
@@ -402,7 +417,11 @@ func (e *Email) ForwardBuildBytes(ctx *context.Context, sender *models.User) []b
 	h.SetAddressList("Sender", senderAddress)
 	h.SetAddressList("To", to)
 	h.SetText("Subject", e.Subject)
-	h.SetMessageID(fmt.Sprintf("%d@%s", e.MessageId, config.Instance.Domain))
+	if e.MsgID != "" {
+		h.SetMessageID(e.MsgID)
+	} else {
+		h.SetMessageID(fmt.Sprintf("%d@%s", e.MessageId, config.Instance.Domain))
+	}
 	if len(e.Cc) != 0 {
 		cc := []*mail.Address{}
 		for _, user := range e.Cc {
@@ -553,7 +572,11 @@ func (e *Email) BuildBytes(ctx *context.Context, dkim bool) []byte {
 	} else {
 		h.SetDate(time.Now())
 	}
-	h.SetMessageID(fmt.Sprintf("%d@%s", e.MessageId, config.Instance.Domain))
+	if e.MsgID != "" {
+		h.SetMessageID(e.MsgID)
+	} else {
+		h.SetMessageID(fmt.Sprintf("%d@%s", e.MessageId, config.Instance.Domain))
+	}
 	h.SetAddressList("From", from)
 	h.SetAddressList("Sender", from)
 	h.SetAddressList("To", to)

server/listen/imap_server/session_fetch.go

@@ -88,8 +88,13 @@ func buildEnvelope(email *response.EmailResponseData, traEmail *parsemail.Email)
 		replyTo = from
 	}
 
-	// Message-ID
-	messageID := fmt.Sprintf("<%d@%s>", email.Id, config.Instance.Domain)
+	// Message-ID: use the persisted value so IMAP clients see a stable ID on every sync.
+	var messageID string
+	if email.MsgID != "" {
+		messageID = fmt.Sprintf("<%s>", email.MsgID)
+	} else {
+		messageID = fmt.Sprintf("<%d@%s>", email.Id, config.Instance.Domain)
+	}
 
 	return &imap.Envelope{
 		Date:      email.CreateTime,

server/listen/smtp_server/read_content.go

@@ -221,6 +221,11 @@ func saveEmail(ctx *context.Context, size int, email *parsemail.Email, sendUserI
 		return nil, nil, nil
 	}
 
+	msgID := email.MsgID
+	if msgID == "" {
+		msgID = parsemail.GenerateMsgID(config.Instance.Domain)
+	}
+
 	modelEmail := models.Email{
 		Type:         cast.ToInt8(emailType),
 		Subject:      email.Subject,
@@ -242,6 +247,7 @@ func saveEmail(ctx *context.Context, size int, email *parsemail.Email, sendUserI
 		Status:       cast.ToInt8(email.Status),
 		CreateTime:   time.Now(),
 		CronSendTime: time.Now(),
+		MsgID:        msgID,
 	}
 
 	_, err := db.Instance.Insert(&modelEmail)
@@ -252,6 +258,7 @@ func saveEmail(ctx *context.Context, size int, email *parsemail.Email, sendUserI
 
 	if modelEmail.Id > 0 {
 		email.MessageId = cast.ToInt64(modelEmail.Id)
+		email.MsgID = modelEmail.MsgID
 	}
 	// 收信人信息
 	var users []*models.User

server/models/email.go

@@ -30,6 +30,7 @@ type Email struct {
 	Error        sql.NullString `xorm:"error text comment('投递错误信息')" json:"error"`
 	SendDate     time.Time      `xorm:"send_date comment('投递时间')" json:"send_date"`
 	CreateTime   time.Time      `xorm:"create_time created" json:"create_time"`
+	MsgID        string         `xorm:"msg_id varchar(255) notnull default('') comment('RFC-compliant Message-ID, generated once on creation')" json:"msg_id"`
 }
 
 func (d *Email) TableName() string {