Our mail servers do not have any configuration for rewriting the received email's subject line. Our mail servers when detected a spam email, it will be bouncing the mails back to the sender directly informing them that the mail has failed to reach you due to the mail being suspected as spam.
You will need to check:
a. Whether if the user has any 3rd party softwares (Example: Anti-Virus with Anti-Spam features) that could be rewriting the subject line
b. Whether if the user's network has any 3rd party appliances that could be rewriting the subject line
c. Based on our past experiences with the "[Spam-pop3]", it would typically be due to the Spam Filtering that is included in the Singtel Corporate Broadband plan and this require you to be raising a case with Singtel for them to check further
