A number of BT Business customers were blacklisted by MessageLabs' anti-spam products on Monday and Tuesday, although the ISP says the issue has now been resolved.
"BT Business is aware that some customers have reported 'bouncebacks' when sending emails to people who are using MessageLabs' anti-spam solutions," BT Business said in a statement on Wednesday. "The problem has now been fixed. We apologise for any inconvenience caused."
There were a number of complaints about the bouncebacks on the BT Business Customer forum this week, with some people suggesting that the problem began at the weekend. However, a BT spokesperson said not many customers had been affected by the issue.
"BT has one million small business customers," said the spokesperson. "As a proportion of that, only a small number of people were affected."
Security company MessageLabs, which is part of Symantec, said in a statement on Wednesday that it was not responsible for the blacklisting of BT customers' IP addresses.
"Any issues with BT customers' email traffic are not a result of any MessageLabs service issue or change," MessageLabs said. "BT should be able to provide further clarification on the incident, which we understand is a result of another security vendor listing some of BT's IPs for spamming, which BT was notified about."
According to customer forum posts, BT technical support told some customers that BT's refusal to block port 25 TCP/IP traffic had led to it being blocked by UCEProtect, whose blacklist is used by a number of security companies to block spam. Port 25 monitoring and blocking is an established anti-spam measure.
BT Business forum moderator Frances Midgley conceded on the forum that IP addresses belonging to BT customers had been blacklisted, but did not name UCEProtect.
"We are aware that an organisation has chosen to 'blacklist' IP addresses which include those provided to BT customers, as well as those of other organisations, alleging that some of these customers' computers are being used to create 'spam' email," wrote Midgley. "However, most cases of blacklisting that are reported to us have turned out to be a technical issue that we can resolve."