Telecom says it has isolated the Yahoo! Xtra email accounts which have been hacked in a recent cyber attack.
The company says about 5% of email accounts have been corrupted and used by hackers to send infected messages to third parties.
Telecom plans to send email messages to the affected customers over the next few days telling them to change their password to prevent the account being used in this way.
The messages will not include any links and will not ask users to provide any personal information.
Telecom says users who have clicked links in suspicious emails have not infected their computers with malicious software.
But it says they should change their password to prevent hackers from using their account to send more spam emails.
The company outsourced Xtra to service provider Yahoo! in 2007.