It started as a marketing gimmick, meant to draw attention to security software firm Inform's products in advance of this week's CeBit IT trade fair. But a few recipients took the German company's direct mail pitch - a blackmail letter from a shadowy underground organisation - all too seriously.

"We know everything. Follow our instructions, or else you're finished. No cops, no tricks!" read the letter, which was sent on 6 March to 2,400 potential customers on Inform's mailing list. The text, in the crooked typeface of an old manual typewriter, is followed by the rubber-stamp seal of the fictitious 'Front for International Data Protection'.

But some readers were not amused. "About five percent took it seriously," said Felicia Krenek, Inform's marketing director. These people contacted their company security departments or the police. The public prosecutor's office in Cologne opened an investigation.

The problem, she said, was that despite all Inform's efforts the second half of the mailing didn't arrive on time. Another mass mailing in the same style went out on 8 March. This one read: "Come to the meeting point at CeBit, Hall 2, at Inform AG... This is your only chance! Bring your data to safety!"

"It was mailed on a Thursday, and a lot of people didn't receive it until the following Tuesday," said Krenek. "We hadn't expected it to take that long."

As soon as the company learned of the confusion, it sent a third letter - this time apologising - to all recipients.

"Please excuse us if we disturbed you. Of course we will no longer use or send this mailing," it reads, this time in a plain, modern font, under Inform's own logo.

As for the legal consequences of the gag, Inform doesn't expect to receive any threatening letters in its own mailbox. "Our lawyers tell us the investigation will probably be dropped," said Krenek. Oops.