Thousands of Xtra customers have had to change their security settings to continue accessing their accounts though third-party software.
An additional layer of security was introduced yesterday for the Xtra email system by Telecom and Yahoo after 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 yesterday.
Users who did not update their settings may find themselves locked out when trying to access their email through third party software such as Microsoft Outlook or through Android devices.
About 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.