Large newspaper advertisements from NTT DoCoMo Inc. are nothing new in Japan. The company routinely shouts about the features and benefits of I-mode, its popular wireless Internet service, although a series that appeared last week highlighted something that is becoming a major annoyance for many of its users and attracting the interest of the government: unwanted junk e-mail, or spam.

I-mode telephones come set up with an e-mail address that matches the handset's telephone number, something like 09012345678@docomo.ne.jp. This makes them easy targets for bulk mailers who can send millions of e-mail messages from Internet-connected computers to random I-mode address for no charge.

Users, unless they change their address to something else, are left to wade through a mountain of spam every time they open their e-mail and, because of I-mode's charging structure, have to foot the bill for downloading such mail to their telephones. All of this has DoCoMo's user's, who collectively send 80 million messages and receive 100 million messages per day, complaining to DoCoMo.

I-mode user Rie Katsurada said she used to receive five to ten junk mails a day, and automatically had to pay for them.

"At first, most of the junk mails were advertisements for game sites or personals sites," Katsurada said.

One time she accessed a game site advertised in a junk mail, and a few days later suddenly started getting lots of spam advertising adult sites.

"I got very annoyed by three mails that came in at once and returned a mail to one of the senders saying, 'Don't send this any more,'" she said.

A few days later, she received a phone call from a stranger who told her that he had found her name and cell phone number on an adult site and wanted to have an affair with her. She immediately changed her phone number.

"It is very possible that the sender harassed her in retaliation, as similar cases have been reported," said Akira Nakamura, who runs the Trouble Task Force for Internet Users web site and offers advice to users facing such problems.

So far DoCoMo's response has been minimal. The newspaper advertisements that ran in several major Japanese national newspapers had diagrams and instructions showing users how to change their e-mail addresses and how to block e-mail from certain senders.

Keiji Tachikawa, DoCoMo's president and chief executive officer, repeated the advice earlier this week.

"We do realize that this is a problem," he said at a news conference. "Basically do not use your phone number as your e-mail address. I have myself received spam mail and I have changed my e-mail address. I do realize this is inconveniencing our users but all I can really do is ask our users to change their e-mail."

But the company may have to do better than that.

The government is now looking into the issue of unwanted e-mail after the number of complaints received by the Electronic Information/Communication Division of the Ministry of Public Management, Home Affairs, Posts and Telecommunications' (MPHPT) regarding junk e-mails via wireless Internet services suddenly increased in March and April, according to Yoshitaka Yano, a counselor of the division at MPHPT.

The MPHPT has requested all domestic mobile telecommunication carriers to take measures to prevent their customers from receiving unwanted spam mails. The request was issued to DoCoMo and its competitors, including DDI Corp., which operates under the Au and Tu-Ka brand names, and J-Phone Communications Co. Ltd.

DoCoMo concedes that asking users to change their e-mail addresses is not a sufficient response to the problem of junk mails and the company is in discussion on the issue, according to Norio Hasegawa, a spokesman for DoCoMo. The company is receiving up to 200 inquiries daily from its users regarding the matter, he said.

"Setting the cell phone number as an e-mail address has been done because it is easy for a user to start the e-mail service immediately after the registration," Hasegawa said. "And we cannot track down those senders because the privacy of communication is protected by the (Japanese) constitution."

Other carriers do not suffer to the same degree as DoCoMo however.

DDI, which has the second largest market share in the wireless Internet market, uses a different system. Since the launch of its EZWeb service, it has equipped handsets with blocking filter functions and has been making its customers create their own e-mail addresses when they start the service, according to Hiroshi Ishihara, a spokesman for DDI.

"But we still have ten or more inquiries a month from our customers regarding junk mails and try to take care of them," Ishihara said.