"I know that our largest client, a major Hawke's Bay firm, is so frustrated they are looking to move all business away from both Xtra and Telecom," he said. "I am aware of numerous emails I have sent which are not being delivered.
"The cost to our business in lost time and opportunities has been large. Today we received three emails which were sent last Wednesday."
He said it would be better if all emails, not some, were blocked.
"We don't know what is going and what is coming - you organise your business around emails. I normally get around 150 emails a day and at the moment that is about 80 or 90. I don't know whether people are sending them or not.
"It is a real issue from a business perspective and certainly we are seriously looking at whether we are continuing with Xtra."
Mr Irvine said customers still experiencing email delays should contact customer support teams "so we can investigate".
Mr Epplett said Xtra was trying to hide the seriousness of the situation.
"It is a huge problem and we can't get Xtra to really admit that they have a big problem."
Hawke's Bay IT consultant Haig Flashoff, of Mint Technology, said at least a dozen Hawkes Bay businesses were seriously affected.
"I was informed on Monday that there have been an additional five secondary mail servers set up at Xtra in order to attempt to resolve the issue," he said.
"While talking with Xtra's CTS helpdesk yesterday, I was able to demonstrate the difference between sending using Xtra's servers, internal DNS, and a third party SMTP host to a Telecom hosted domain. This clearly showed that sending email via a Telecom connection - even using our own DNS - to a Telecom hosted domain, was sporadically causing significant delays.
"The response I received to this demonstration was, 'We recommend you do not use our mail servers to send your email'."
Mr Flashoff said using an ISP's SMTP host was once standard practice and there were "a considerable amount of businesses still doing this".
"In recommending that people do not use their email servers to send email, Xtra is showing a complete lack of responsibility to their paying customers who have become reliant on stable and reliable email services.
"It is my recommendation that people look closely at their email vulnerabilities, and assess the risk associated with email downtime.
"Given the current problems at Xtra, their lack of communication, responsibility and inability to resolve the issues in a timely fashion, I would not recommend their services to anyone, moving forward."
Mr Irvine said Telecom clients were not told of the email malfunction directly "but there was information on our website".
"We apologise to anyone having issues - we are doing the best that we can to try and get them resolved."
In January, customers using Xtra's Yahoo service were told their username and password were invalid when they tried to log in. In 2007, Xtra customers were given one week's free internet as an apology for email problems and Telecom donated $1 million to charity.
Mr Epplett said an Xtra technician had told him the problem had not been fixed.
