It's a simple issue, but may require more understanding that we want to know about. The message "relaying mail to pacific.net.sg is not allowed" means you can't send email through that server when you are not on their network.
It's their policy and nothing on your end will change it.
There are times when i send emails through my XP Outlook, while sending, an error message prompted:
Undeliverable: message title
Message can not reach some or all of the intended recipients,
Subject: message title,
550 relaying mail to pacific.net.sg is not allowed.
Any ideas? Thanks!