This news will come as an embarrassment to MessageLabs, which is promoting a number of anti-spam solutions to users of its mail-filtering service, and the company has moved quickly to rectify the problem.
Silicon.com first learned of the problem when a reader who is a MessageLabs customer tipped us off about mail delivery failure notifications he had received from AOL.
A spokesman for AOL told silicon.com: "We blocked a small proportion of MessageLabs' mail servers after they triggered our spam filter. We are removing the dynamic block at this time as MessageLabs has resolved their issue. We are sorry if any Internet user, including your reader, was inconvenienced, but this kind of action is vital to protect members from spam."
Matt Sergeant, senior anti-spam technologist at MessageLabs, suggested this was a freak occurrence which involved a coming together of a number of unfortunate circumstances.
He told silicon.com: "We have more than 6,000 customers, and every now and again a customer will install a service pack or reconfigure a server in such a way that it exposes a flaw, such as an open relay in their server.
"We scan customers' servers every 12 hours, looking for things such as open relays. Unfortunately the spammers are out there scanning servers looking for these things as well and sometimes they are scanning at a time when we are not, or at a more frequent rate. In this case it appears the spammers may have found an insecure server and abused it."
Sergeant confirmed that MessageLabs had spotted the problem within hours of it arising, and had closed down the server to stop further abuse -- but not before AOL had also spotted it and put the block in place.
"To AOL it would have appeared that the spam was coming from our server, so they blocked mail travelling through that sever," he said. "But the issue is now resolved."
Sergeant said any affected customers had been contacted and informed of the problem.
who gives a rats arse about aol?