I found the problem. It is because email deliver from our server to  yahoo.com.sg was treated as spam message. It is your email server which block the mail message and return back to our server.
May be you have asked our server sent too many messages to you in a short time, and your mail server  treat our message as spam  messages.

The following are message from yahoo.com.sg: I change your email address to XXX@yahoo.com.sg
This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  XXX @yahoo.com.sg
    SMTP error from remote mail server after initial connection:
    host d.mx.mail.yahoo.com []: 553 Mail from not allowed - 5.7.1 [BL23] Connections not accepted from IP addresses on Spamhaus XBL; see http://postmaster.yahoo.com/550-bl23.html [550]