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.
It was currently unknown how many had changed their settings before the 3pm deadline.
All customers were still able to access their accounts using webmail at xtramail.co.nz.
"We have had a really good response from customers over the weekend changing their settings,'' the spokeswoman said.
"It only takes about five minutes and there are really clear instructions on our website. However, customers will need to be aware that they'll have to do it for all of their devices.''
Users who have already changed their settings to SSL (Secure Sockets Layer) or who use their account via webmail or Yahoo mobile applications do not need to change.
The added encryption is designed to make the email service more secure, particularly on public wifi connections.
Xtra was plagued by spammers last year, with more than 60,000 users sent malicious spam in one attack. Another attack forced some email accounts to be locked until users changed their password.
* Customers can visit www.telecom.co.nz/SSL for detailed instructions on making the setting change, or call 0800 268 268.
* For more information on the SSL update, go to www.telecom.co.nz/ssl.
* Information for staying safe online is available at www.telecom.co.nz/techinasec or via Netsafe at www.netsafe.org.nz.