sending mail after making order

After I created an order by admin or any other users, and submitted the order. I got the following message.

* warning: mail() [function.mail]: SMTP server response: 503 5.0.0 Need RCPT (recipient) in C:\xampp\htdocs\drupal1\includes\mail.inc on line 194.
* Unable to send e-mail. Please contact the site administrator if the problem persists.

My smtp setting is:
1) In php.ini, I was using external smtp server from ISP to send email. and i was using the email account from the ISP as well.

[mail function]
; For Win32 only.
SMTP = smtpo.xxx.net
smtp_port = 25

; For Win32 only.
sendmail_from = myaccount@xxx.net

2) In Site information, the E-mail address filed was using the same email address.

I can receive the email from the store if I use an registered account having a web mail (say. gmail). but the error message from the above is still existed.
I can't find the reason of this issue, would you please help me to solve it out? Thanks!