Spark customers with Xtra Mail accounts will have to pay $5.95 a month to use them from mid-May.

It's citing rising maintenance costs as the reason.

Spark product director Tessa Tierney says the company considered scrapping the service altogether.

She says it's hard to say what its future will be.

"So it'll be interesting to see over the next few weeks - and months - what decisions customers make. But at this stage, I couldn't tell you how many will decide to leave."

LISTEN ABOVE