Junk email has been causing havoc for internet users and service providers since Saturday, with around 37,500 ihug customers experiencing the most serious delays.
Ihug spokeswoman Cherie Lacey said up to half of the company's 75,000 customers were experiencing 24-hour delays in receiving their emails.
But the ISP's email service has been choked over the past few days due to "a massive outbreak in viruses and spam".
Ms Lacey said the junk emails disguised as images managed to bypass the ihug filter.
As a result, ihug became overloaded and emails sent to customers over the weekend took an extra day.
The company installed new software to combat the problem and hoped the delays would be resolved within a couple of days.
"It is a constant struggle to keep out spam, there are little wars going on out there in cyberspace to keep it out."
Ms Lacey said once the email backlog was cleared, ihug would be installing new hardware to combat the ongoing problem of junk email and viruses.
About 2000 business customers were affected.
One ihug business customer, who works from his home, said he was unable to deliver a presentation in Auckland yesterday because he had not received the email containing the material he needed.
The Mt Albert graphic designer, who did not want to be named, had no idea how much the delay would cost in lost business.
He did not know how many emails were due to arrive, making it difficult to guess how much business he had lost. ihug had not indicated when the problem would be fixed.
Internet service provider Maxnet's customers also suffered delays in receiving emails.
Helpdesk manager Jay Best said the company had experienced a 40 per cent increase in spam within the past month. Again, junk emails disguised as images had bypassed his company's filtering systems.
Mr Best said Maxnet customers encountered delays of up to two hours and the company had installed new software to recognise the junk emails and get rid of them.
"In one extreme case we had a customer who received 80,000 junk emails last month."