BTclick's mail servers crashed this morning, preventing thousands of small businesses from sending or receiving email.
Servers went offline at 11am with a mysterious fault and by close of play the system was still down.
"Engineers are unable to tell what the problem is at present, although the theory is that a rogue mailbox somehow managed to remove access to both the main servers and their back-ups," a spokeswoman for the company said.
This was, however, only one of several possibilities, she added.
BT said that engineers were re-installing software and that it hoped the servers would be back online by close of business. But by 5.30pm, the spokeswoman said that the problem may not be fixed until the following day.
BTclick's helpdesk has been flooded with calls about the fault, although at this point technical engineers were able to provide no further information.
"We have posted an automatic message on the helpdesk phone to notify users," the spokeswoman said.
Thousands of small-business customers use the broadband services provided by BTclick, which is administered by BTOpenworld.
Cotton seedling freezes to death as Chang'e-4 shuts down for the Moon's 14-day lunar night
Fortnite easily out-earns PUBG, Assassin's Creed Odyssey and Red Dead Redemption 2 in 2018
Meteor showers as a service will be visible for about 100 kilometres in all directions
Saturn's rings only formed in the past 100 million years, suggests analysis of Cassini space probe data
New findings contradict conventional belief that Saturn's rings were formed along with the planet about 4.5 billion years ago