diff --git a/services/mailer/incoming/incoming.go b/services/mailer/incoming/incoming.go
index 555cdfee8b..6530b7cc60 100644
--- a/services/mailer/incoming/incoming.go
+++ b/services/mailer/incoming/incoming.go
@@ -219,7 +219,7 @@ loop:
 			}
 
 			err := func() error {
-				if handledSet.Contains(msg.SeqNum) {
+				if isAlreadyHandled(handledSet, msg) {
 					log.Debug("Skipping already handled message")
 					return nil
 				}
@@ -282,6 +282,11 @@ loop:
 	return nil
 }
 
+// isAlreadyHandled tests if the message was already handled
+func isAlreadyHandled(handledSet *imap.SeqSet, msg *imap.Message) bool {
+	return handledSet.Contains(msg.SeqNum)
+}
+
 // isAutomaticReply tests if the headers indicate an automatic reply
 func isAutomaticReply(env *enmime.Envelope) bool {
 	autoSubmitted := env.GetHeader("Auto-Submitted")
diff --git a/services/mailer/incoming/incoming_test.go b/services/mailer/incoming/incoming_test.go
index 5d84848e3f..f2bb7fc498 100644
--- a/services/mailer/incoming/incoming_test.go
+++ b/services/mailer/incoming/incoming_test.go
@@ -7,10 +7,24 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/emersion/go-imap"
 	"github.com/jhillyerd/enmime"
 	"github.com/stretchr/testify/assert"
 )
 
+func TestNotHandleTwice(t *testing.T) {
+	handledSet := new(imap.SeqSet)
+	msg := imap.NewMessage(90, []imap.FetchItem{imap.FetchBody})
+
+	handled := isAlreadyHandled(handledSet, msg)
+	assert.Equal(t, false, handled)
+
+	handledSet.AddNum(msg.SeqNum)
+
+	handled = isAlreadyHandled(handledSet, msg)
+	assert.Equal(t, true, handled)
+}
+
 func TestIsAutomaticReply(t *testing.T) {
 	cases := []struct {
 		Headers  map[string]string
@@ -95,6 +109,32 @@ func TestGetContentFromMailReader(t *testing.T) {
 	assert.Equal(t, "attachment.txt", content.Attachments[0].Name)
 	assert.Equal(t, []byte("attachment content"), content.Attachments[0].Content)
 
+	mailString = "Content-Type: multipart/mixed; boundary=message-boundary\r\n" +
+		"\r\n" +
+		"--message-boundary\r\n" +
+		"Content-Type: multipart/alternative; boundary=text-boundary\r\n" +
+		"\r\n" +
+		"--text-boundary\r\n" +
+		"Content-Type: text/plain\r\n" +
+		"Content-Disposition: inline\r\n" +
+		"\r\n" +
+		"mail content\r\n" +
+		"--text-boundary--\r\n" +
+		"--message-boundary\r\n" +
+		"Content-Type: text/plain\r\n" +
+		"Content-Disposition: inline; filename=attachment.txt\r\n" +
+		"\r\n" +
+		"attachment content\r\n" +
+		"--message-boundary--\r\n"
+
+	env, err = enmime.ReadEnvelope(strings.NewReader(mailString))
+	assert.NoError(t, err)
+	content = getContentFromMailReader(env)
+	assert.Equal(t, "mail content", content.Content)
+	assert.Len(t, content.Attachments, 1)
+	assert.Equal(t, "attachment.txt", content.Attachments[0].Name)
+	assert.Equal(t, []byte("attachment content"), content.Attachments[0].Content)
+
 	mailString = "Content-Type: multipart/mixed; boundary=message-boundary\r\n" +
 		"\r\n" +
 		"--message-boundary\r\n" +