Customers locked out of their email accounts all day on Wednesday should have access again by the morning, Vodafone says.
Clearnet and Paradise email customers are having trouble logging into their webmail services and there are problems syncing with Outlook and other third-party email clients.
Vodafone said there were also delays sending and receiving emails.
A spokeswoman said it was not clear how many customers had been affected.
“But the ones that have are understandably very frustrated,” she said.
“It’s due to a hardware issue that came up unexpectedly this morning, affecting one of our email servers. There’s an urgent team working on the problem and we are expecting it to be resolved overnight.”
Customers have taken to the company’s Facebook page to complain.
John Wierenga wrote: “I first phoned Vodafone faults about an issue with email sign in at 4.00am this morning- still not fixed 16 hours later!. This is really frustrating- how the heck are you supposed to run a business with no email send or receive?”
He said affected customers should be offered compensation.