Thousands of Xtra customers will be unable to access their email account from Monday until they change their email security settings.
An additional layer of security was being added to Xtra email after a spate of security issues, Telecom and Yahoo said.
The additional encryption setting would be applied from Monday, making email more secure, particularly on public wifi connections, the companies said.
That security layer would affect third party clients who use programmes such as Microsoft Outlook or use Android devices to check their messages - about a third of Telecom's customers, a Telecom spokeswoman said.
"Some of those people will already have the settings changed, but there will be a proportion who haven't done that.
"We're talking thousands (of clients),'' she said.
The Telecom system would not be affected by thousands logging on at the same time to change their settings.
Because it was third party clients, Telecom would not be able to have a message show up when users logged on to check their emails, the spokeswoman said.
But the company had been trying to contact all those affected by email, SMS or phone calls.
The email service had been badly hit by spammers, forcing some email accounts to be locked down until users changed their password.
Users who have already changed their settings to SSL (Secure Sockets Layer) or who use their account via webmail or Yahoo mobile applications will not need to change.
Telecom Retail chief executive Chris Quin said the sooner the additional security can be applied, the better.
"We've been contacting affected Xtra users by phone, SMS, email and post, and while we're pleased to see a number of customers have chosen to make the change themselves, there is still a way to go.
"We've made the call that, when it comes to their online security, our customers want us to do all we can protect them and they don't want us to muck around with it.''
He said Telecom would continue to roll out more security on the Xtra email platform.
"We're confident this setting change will further assist that effort, but we encourage all email users to keep their wits about them and consider how they can play a role in preventing the circulation of spam.
"This means never clicking suspicious links contained in emails, which can perpetuate spammers' activity, even if the email appears to come from a friend or family member.''
Users were advised to regularly update account settings with a strong, difficult-to-predict password, and never to use the same password for their online banking, their email and their social media profiles.
* 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.
* If users had problems, they could access their mail through the Yahoo email app or a web browser.