diff --git a/internal/smtp/http.go b/internal/smtp/http.go
index d610265..e752081 100644
--- a/internal/smtp/http.go
+++ b/internal/smtp/http.go
@@ -101,14 +101,13 @@ func (h *smtpHTTPHandler) handleMessageIndex(w http.ResponseWriter, r *http.Requ
 	if headerSearchRgx == nil {
 		receivedMessages = h.server.ReceivedMessages()
 	} else {
-		// FIXME: This does not preserve the correct indexes!
 		receivedMessages = h.server.SearchByHeader(headerSearchRgx)
 	}
 
 	messagesOut := make([]any, 0)
 
-	for i, msg := range receivedMessages {
-		messagesOut = append(messagesOut, buildMessageBasicMeta(msg, i))
+	for _, msg := range receivedMessages {
+		messagesOut = append(messagesOut, buildMessageBasicMeta(msg))
 	}
 
 	out := make(map[string]any)
@@ -124,7 +123,7 @@ func (h *smtpHTTPHandler) handleMessageMeta(w http.ResponseWriter, r *http.Reque
 		return
 	}
 
-	out := buildMessageBasicMeta(msg, idx)
+	out := buildMessageBasicMeta(msg)
 
 	out["body_size"] = len(msg.Body())
 
@@ -169,14 +168,13 @@ func (h *smtpHTTPHandler) handleMultipartIndex(w http.ResponseWriter, r *http.Re
 	if headerSearchRgx == nil {
 		multiparts = msg.Multiparts()
 	} else {
-		// FIXME: This does not preserve the correct indexes!
 		multiparts = msg.SearchPartsByHeader(headerSearchRgx)
 	}
 
 	multipartsOut := make([]any, 0)
 
-	for i, part := range multiparts {
-		multipartsOut = append(multipartsOut, buildMultipartMeta(part, i))
+	for _, part := range multiparts {
+		multipartsOut = append(multipartsOut, buildMultipartMeta(part))
 	}
 
 	out := make(map[string]any)
@@ -201,7 +199,7 @@ func (h *smtpHTTPHandler) handleMultipartMeta(
 		return
 	}
 
-	handlerutil.RespondWithJSON(w, http.StatusOK, buildMultipartMeta(part, partIdx))
+	handlerutil.RespondWithJSON(w, http.StatusOK, buildMultipartMeta(part))
 }
 
 func (h *smtpHTTPHandler) handleMultipartBody(
@@ -256,16 +254,16 @@ func retrievePart(w http.ResponseWriter, msg *ReceivedMessage, partIdx int) *Rec
 	return msg.Multiparts()[partIdx]
 }
 
-func buildMessageBasicMeta(msg *ReceivedMessage, idx int) map[string]any {
+func buildMessageBasicMeta(msg *ReceivedMessage) map[string]any {
 	return map[string]any{
-		"idx":        idx,
+		"idx":        msg.Index(),
 		"receivedAt": msg.ReceivedAt(),
 	}
 }
 
-func buildMultipartMeta(part *ReceivedPart, partIdx int) map[string]any {
+func buildMultipartMeta(part *ReceivedPart) map[string]any {
 	out := map[string]any{
-		"idx":       partIdx,
+		"idx":       part.Index(),
 		"body_size": len(part.Body()),
 	}
 
diff --git a/internal/smtp/message.go b/internal/smtp/message.go
index 08ce2bc..848e916 100644
--- a/internal/smtp/message.go
+++ b/internal/smtp/message.go
@@ -16,6 +16,8 @@ import (
 
 // ReceivedMessage contains a single email message as received via SMTP.
 type ReceivedMessage struct {
+	index int
+
 	smtpFrom       string
 	smtpRcptTo     []string
 	rawMessageData []byte
@@ -34,6 +36,8 @@ type ReceivedMessage struct {
 // ReceivedPart contains a single part of a multipart message as received
 // via SMTP.
 type ReceivedPart struct {
+	index int
+
 	headers textproto.MIMEHeader
 	body    []byte
 }
@@ -45,6 +49,7 @@ type ReceivedPart struct {
 // If a maxMessageSize of 0 is given, this function will default to using
 // DefaultMaxMessageSize.
 func NewReceivedMessage(
+	index int,
 	from string, rcptTo []string, rawMessageData []byte, receivedAt time.Time,
 	maxMessageSize int64,
 ) (*ReceivedMessage, error) {
@@ -63,6 +68,7 @@ func NewReceivedMessage(
 	}
 
 	msg := &ReceivedMessage{
+		index:          index,
 		smtpFrom:       from,
 		smtpRcptTo:     rcptTo,
 		rawMessageData: rawMessageData,
@@ -92,7 +98,7 @@ func NewReceivedMessage(
 
 		r := multipart.NewReader(bytes.NewReader(msg.body), boundary)
 
-		for {
+		for i := 0; ; i++ {
 			rawPart, err := r.NextPart()
 			if err != nil {
 				if errors.Is(err, io.EOF) {
@@ -102,7 +108,7 @@ func NewReceivedMessage(
 				}
 			}
 
-			part, err := NewReceivedPart(rawPart, maxMessageSize)
+			part, err := NewReceivedPart(i, rawPart, maxMessageSize)
 			if err != nil {
 				return nil, fmt.Errorf("could not parse message part: %w", err)
 			}
@@ -150,7 +156,7 @@ func (m *ReceivedMessage) SearchPartsByHeader(re *regexp.Regexp) []*ReceivedPart
 // Incoming data is truncated after the given maximum message size.
 // If a maxMessageSize of 0 is given, this function will default to using
 // DefaultMaxMessageSize.
-func NewReceivedPart(p *multipart.Part, maxMessageSize int64) (*ReceivedPart, error) {
+func NewReceivedPart(index int, p *multipart.Part, maxMessageSize int64) (*ReceivedPart, error) {
 	if maxMessageSize == 0 {
 		maxMessageSize = DefaultMaxMessageSize
 	}
@@ -161,6 +167,7 @@ func NewReceivedPart(p *multipart.Part, maxMessageSize int64) (*ReceivedPart, er
 	}
 
 	part := &ReceivedPart{
+		index:   index,
 		headers: p.Header,
 		body:    body,
 	}
@@ -184,6 +191,10 @@ func (m *ReceivedMessage) Headers() mail.Header {
 	return m.headers
 }
 
+func (m *ReceivedMessage) Index() int {
+	return m.index
+}
+
 func (m *ReceivedMessage) IsMultipart() bool {
 	return m.isMultipart
 }
@@ -215,3 +226,7 @@ func (p *ReceivedPart) Body() []byte {
 func (p *ReceivedPart) Headers() textproto.MIMEHeader {
 	return p.headers
 }
+
+func (p *ReceivedPart) Index() int {
+	return p.index
+}
diff --git a/internal/smtp/server.go b/internal/smtp/server.go
index b041849..0591a1b 100644
--- a/internal/smtp/server.go
+++ b/internal/smtp/server.go
@@ -153,10 +153,13 @@ func (s *session) Data(r io.Reader) error {
 	s.server.mutex.Lock()
 	defer s.server.mutex.Unlock()
 
+	idx := len(s.server.receivedMessages)
 	now := time.Now()
 
 	logrus.Infof("SMTP: Receiving message from %s to %v at %v", s.from, s.rcptTo, now)
-	msg, err := NewReceivedMessage(s.from, s.rcptTo, rawData, now, s.server.maxMessageSize)
+	msg, err := NewReceivedMessage(
+		idx, s.from, s.rcptTo, rawData, now, s.server.maxMessageSize,
+	)
 	if err != nil {
 		errWrapped := fmt.Errorf("error constructing ReceivedMessage in SMTP server: %w", err)
 		logrus.Error("SMTP:", errWrapped) // this is logged in our server