Add To: header when constructing emails (#28507)

#28032

changes/28032-email-to-header

@@ -0,0 +1 @@
+- Fixed missing To: email header

server/mail/mail.go

@@ -78,7 +78,13 @@ func getMessageBody(e fleet.Email, f fromFunc) ([]byte, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to obtain from address: %w", err)
 	}
-	msg := []byte(subject + from + mime + content + "\r\n" + string(body) + "\r\n")
+	to := ""
+	if len(e.To) == 1 {
+		to = "To: " + e.To[0] + "\r\n"
+	} else if len(e.To) > 1 {
+		to = "To: undisclosed-recipients\r\n"
+	}
+	msg := []byte(subject + from + to + mime + content + "\r\n" + string(body) + "\r\n")
 	return msg, nil
 }