Spark will be moving its Xtra email service to New Zealand-owned email provider SMX.
Customers who use @xtra.co.nz email addresses need to give Spark permission to move their data to the new platform.
The transfer of Xtra customer data will take three months from January next year and customers can begin the process from today. Email addresses won't need to be changed.
SMX is based in Auckland and rich lister Sam Morgan is one of its major shareholders.
Spark Home, Mobile and Business CEO, Jason Paris said 11,000 Xtra business customers were moved to SMX last year.
"We know that email is a critically important service to our customers and we are dedicated to making this transition as seamless as possible for our users," Paris said.
"SMX is one of the leading cloud email providers in New Zealand and we are confident in their ability to bring customers' email data back home, safely and securely."
The company will be working to "ensure the migration runs as smoothly as possible," with most work managed in off-peak hours at night, a statement said.
Customers will need to opt-in to the migration process as soon as possible to ensure their contacts, calendar entries and emails are migrated from Yahoo.
The Xtra email data will be stored at Spark's Takanini data centre.
Customers can go to www.spark.co.nz/email for more information or they can call a dedicated Xtra helpline on 0800 934 348 if they have any questions.