diff --git a/plugins/base/smtp.go b/plugins/base/smtp.go
index 19c9e0b..dc8211e 100644
--- a/plugins/base/smtp.go
+++ b/plugins/base/smtp.go
@@ -138,7 +138,8 @@ func (msg *OutgoingMessage) WriteTo(w io.Writer) error {
 	if msg.InReplyTo != "" {
 		h.Set("In-Reply-To", msg.InReplyTo)
 	}
-	// TODO: set Message-ID
+
+	h.Set("Message-Id", mail.GenerateMessageID())
 
 	mw, err := mail.CreateWriter(w, h)
 	if err != nil {