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.

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Hooker advised customers to exercise caution when opening emails if they don't know where they came from.