The e-mail service, BTClick, which forms part of BT Openworld's business ADSL provision, was down for about 48 hours between 13 and 15 November.
Companies calling the helpline were first told that there was such a mail backlog that they could not be given any estimate of when e-mails would be delivered.
Later callers to the helpdesk were told that the service was fully functional and that any missing e-mails must be "lost".
One customer, IT consultant David Anderson, was told that there were no back-up servers for the e-mail service.
Helpdesk staff told him that gateway upgrades which could remedy this problem will not be implemented until July 2002, Anderson said.
The effects of e-mail on his IT consulting business are typical. "It is vital to our business that we receive e-mail. It is a very important point of contact with existing and potential customers.
"We have been able to trace the senders of many whom we know, and have them re-sent, but those that are coming in from potential new business are lost to us," he said.
BT Openworld was not available for comment at the time of going to press.