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.
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