Spark is seeking clarification from Yahoo as to whether any of its customers had email searched at the behest of United States intelligence agencies.
According to an investigation by Reuters, Yahoo last year used a software program to search its customers' incoming email for specific information provided by US intelligence agencies.
That happened after Yahoo complied with a classified US government demand, it was reported.
Spark is now trying to get more information, including whether Spark's Xtra Mail customers could have been affected.
Yahoo hosts more than 800,000 accounts on behalf of Spark.
"At this time we are still seeking clarification from Yahoo if any Spark Xtra email customers are involved," a spokeswoman told the Herald.
Privacy Commissioner John Edwards said Spark was correct to be making its own inquiries.
"I would be concerned if these kinds of arrangements were occurring without a clear legal authority. But we haven't seen enough in the Reuters report to know the basis of it."
Edwards said there was a big difference between an intelligence agency asking a provider to scan all email accounts, and seeking access to a particular communication when there were legitimate law enforcement reasons to do so.
"They have no reason to believe that any individual is a suspect. They are just going across everything."
Spark will soon migrate its Xtra email accounts back to New Zealand, to be hosted by cloud email provider SMX. Edwards said the Reuters report would fuel such moves worldwide.
"It really undermines confidence in the digital economy."
Last month Spark contacted 131,000 Xtra customers about another scare, after hackers accessed hundreds of millions of Yahoo accounts in late 2014.
They were advised their accounts were potentially affected and to change their password. The Spark spokeswoman said Yahoo had since advised there was no evidence that information from Xtra accounts had been used for malicious activity.