The company said some customers would be unable to log in to their email, using an email client.
"Customers will receive an error when attempting to authenticate. Customers will also experience a delay in receiving email," an advisory said.
Webmail services are also affected. "Some customers will be unable to log in to email via webmail or experience poor performance after logging in," the advisory said.
A third problem area is voice services. "Customers will be unable to check voicemail messages on netphone services," the advisory said.
And finally, the iiNet contact centre has been out of operation for the last five hours. The company says they expect that this problem will be rectified in another four hours.
There is no expected time of resolution for the other issues.
iiNet became Australia's second largest ISP after Telstra when it was acquired by TPG last year.