@Novi_Hidayati, you’ll need to identify which mechanism (SMTP or php’s mail()) you are using for sending, and then begin debugging there.
For example, if your config.inc.php has SMTP turned off, you (or the sysadmin) can look in the local mail server logs for details. If SMTP is turned on, you will need to have that server’s administrator check the logs and report whether mail appears to be sent off-system successfully.
Recipients should check their “Junk” or “Spam” folders in case messages are being caught there. If so, there is usually a way to add the sender to a “safe senders” list.