Thousands of Xtra customers have had to change their security settings by today, in order to continue accessing their accounts though third-party software.
An additional layer of security was being rolled out this afternoon across the Xtra email system by Telecom and Yahoo, following a spate of security issues.
Customers had been contacted via email, phone, or text message since February and the security settings came into effect at 3pm today.
Users who did not update their settings by the 3pm deadline may find themselves locked out when trying to access their email through third party software such as Microsoft Outlook or through Android devices.
Around 140,000 users - a third of Xtra's customer base - had accessed their emails through such software in the past 90 days, a Telecom spokeswoman said.