Some Telecom business customers may find the "scan-to-email" function on their office photocopiers and printers stops working this afternoon.
From 3pm, Telecom will begin blocking customers from accessing their email accounts via third-party clients such as Microsoft Outlook if they haven't turned on a setting called SSL (secure socket layer) encryption.
The block is being put in place to reduce the risk of customers' email data being intercepted before it reaches the servers of Yahoo, which runs Xtra, and is one of a series of planned security improvements to the beleaguered email service.
But Justin Millier, director of Dunedin technology company Deglitch, said there were many situations in which businesses might want to send emails and notifications via a "non-authenticated host", which would not be possible once the block was in place.
The most common was to let office workers send scanned documents via email from multifunction devices, he said.
He forecast businesses would experience a large amount of disruption and said most would likely need help from their printer company to make all the changes they needed to re-enable scan-to-email.
One printer and copier manufacturer had sent out instructions to customers on how to make the change themselves which it did not appear to have tested and which did not work, he said.
Telecom spokeswoman Lucy Fullarton said the company was aware there would be some issues, but believed they would affect only "a very small number of customers".
"The best advice we can give them if they are concerned is to give us a call and we will make sure we put them in touch with someone who can help them," she said.