Some Vodafone customers are facing their fourth day of email delays, while others are reporting a "deluge" of spam in their inboxes.
The issues are affecting Vodafone's Clearnet and Paradise customers, some of whom have been experiencing problems since Tuesday, a company spokeswoman said.
Vodafone's head of technology operations, Ian Hooker, said people using these email services may be experiencing a higher than usual level of spam, "with a particular surge this week".
"This global volume and the measures we are taking to reduce the spam have had a knock-on effect on email performance. We are working around the clock to clear the backlog and reduce the garbage coming into our customers' inboxes," Hooker said.
Hooker advised customers to exercise caution when opening emails if they don't know where they came from.