I have problem with sending email to clients. For debugging purpose, the email archive is enabled and I have confirmed that all emails sent are archived. According to http://www.zen-cart.com/wiki/index.p...Email_Problems , it is said that If it's in the archive, it was handed off to the mailserver. Once the email leaves Zen Cart™, it's at the mercy of your mailserver.

So can I confirm that the problem does not happened due to ZenCart configuration, but resides in my mail hosting site?

This is the settings done at Admin -> Configuration -> Email Options
E-Mail Transport Method : smtpauth
Email Address (Displayed to Contact you) : admin@MYDOMAIN.COM
Email Address (sent From) : admin@MYDOMAIN.COM
SMTP Email Account Mailbox: admin@MYDOMAIN.COM
SMTP Email Account Password : MY_ADMIN_PWD
SMTP Email Mail Host: mail.MYDOMAIN.COM
SMTP Email Mail Server Port: 25

Also for debugging purpose , I have turned on " define('EMAIL_SYSTEM_DEBUG','on'); ". The email content is shown correctly with password given, however, I did receive a warning as shown below, which I guess has no harm but just to raise out its existence.

Warning: Cannot modify header information - headers already sent by (output started at MYDIR/includes/functions/functions_email.php:360) in MYDIR/includes/functions/functions_general.php on line 41

Any clue??