Businesses using Spark's email platform Xtra say it is hindering their productivity.
Speaking to the Herald business owners described the platform as troublesome in that it was restricting work-based emails from being sent and received.
This is not the first time Spark's platform has been criticised, with customers in the past complaining of receiving large quantities of spam emails.
To combat the issue Spark added an additional spam filter to its system which had since caused restrictions on legitimate emails.
One small business owner, who does not wish to be identified, said the platform had restricted incoming and outgoing emails to his business clients.
"We are losing business every day as we cannot reply to these customers... invoices from business to Xtra accounts are not reaching customers either," the man said.
Despite Spark saying it was working to resolve the issue, two business owners said the telecommunications company had not responded to their requests regarding the matter.
In a statement, Spark said a second filter had been added to Xtra Mail to combat spam mail and had caused the restriction of some legitimate emails.
"The filter does not discriminate between business or personal users, it works on an algorithm that flags emails that contain spamming characteristics. These tend to be bulk email senders or when using a generic shared mail platform," it said.
"With the prevalence of spammers, scammers and hackers we work hard to continually provide a solution that meets both needs, however, there is no permanent silver bullet, it's an ongoing fine-tuning exercise."
Spark said just four of its 720,000 Xtra email users were in contact with the company about this issue but its email issues are being widely discussed online, including on technology blogs on Geekzone and Trade Me.
One user posted on Geekzone that emails sent to Xtra accounts had bounced back.
"A friend of mine who owns his own business is sending out invoices and statements at the end of each month, [and] the customers who are with Spark (@xtra.co.nz email address) are not receiving this and a message is being bounced back to the sender advising the message was blocked due to spam content."
Another wrote: "Our domain seems to have been blacklisted by Xtra. We are a small business trying to send emails to our customers.
"I can send two identical emails from the same server, just with different domains, and only one of our domains is blocked (so... not an RBL problem)."
A web developer told the Herald he had done testing and looked into what had caused emails sent to and from Xtra accounts to bounce back. He said he found it was the wording of the emails, not attachments.
"I've done a lot of testing on various emails and the conclusion that I've come to at the moment is that Xtra are blocking some financially-related emails," he said.
"Xtra and Spark have just made their anti-spam overzealous, it's catching things it really shouldn't be."
He said he first became aware of the issues in July.
Spark has recently made changes to its 'spam button' functions as it found customers were using it as a 'delete button' without being aware of the consequences, the company said.
"There are instances where the complex algorithms used by the filters can incorrectly identify and block legitimate emails as spam. These tend to be emails sent in bulk using a generic shared mail platform such as MailChimp. In these cases, we are unable to simply whitelist the entire platform as it will leave the customer vulnerable to unsolicited correspondence."
Spark said it was working to fine-tune its filter to appropriately identify spam.