BTopenworld got its BTclick mail servers back online in the early hours of this morning after the service was knocked out for 18 hours.
Servers went offline at 11am yesterday with a fault caused by a corrupted mailbox.
A BTopenworld spokesman said that not all BTclick customers were affected by the fault, but that there had been a "significant number".
He was expecting a backlog at the site while the servers try to deliver more than 18 hours worth of mail. The service is expected to be back to normal by this afternoon.
"We are currently looking at what happened to see if there is any way this fault could be avoided," the spokesman said.
Thousands of small-business customers use the BTclick broadband services.
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