Telecom admits a number of its customers are still unable to access Xtra webmail.
It follows a major upgrade of 800,000 mailboxes over the weekend.
Telecom spokesman Adrian Littlewood says a fault in the registration process was detected late last night.
He says the problem only affects a small proportion of customers but is complex and he is not sure how long it is going to take to fix.
Mr Littlewood says extra call centre staff are being put on to deal with the increased number of calls.
One Xtra customer claims Telecom did not provide enough information about an email upgrade which affected his business.
Telecom is denying claims it did not let customers know about the outage and says it sent out several emails advising them.
Godfrey Therkleson from Lower Hutt says he received all the notifications that there would be some issue with his email account, but he says none of the warnings prepared him for being left without email for so long.
He says it is incredibly difficult to run a small business without the service because so many of his healthcare business clients in New Zealand and overseas use his current address.
Mr Therkleson says he is also very unhappy with the way Telecom';s support staff handled the issue and is considering changing service providers, but says that is not an easy task.
- NEWSTALK ZB