Senior systems administrator Paul Craig says the service has been undergoing planned maintenance and upgrading which has caused some outages, however a spam attack from overseas hasn't helped.
"For the past week or so we've been hit by Nigerian spammers and they've caused a few problems that we didn't exactly need."
Craig says the spammers are trying a simple approach, sending messages to any possible Nzoomail name.
"They've been stupidly guessing accounts, sending mail to names like firstname.lastname@example.org and so forth. It's causing havoc on the system."
Craig says measures are being taken to stop the spam and these should help prevent further problems.
"Hopefully all issues will be resolved by the end of business [yesterday]".
The Nzoomail service is one of the most popular web-based email offerings in New Zealand, second only to XtraMSN's Hotmail, according to Craig.
"We're upgrading and trying to improve the service. Most of it's behind the scenes, making changes to the architecture, updating our servers and expanding our cluster of servers, that sort of thing."
Craig hopes the changes will ensure fewer problems in the months ahead.