"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.