The company, which has over 41 million subscribers and is one of the largest mobile operators in Japan, will take advantage of a recently-passed law aimed at making filtering of junk mail or spam easier.
The law, drafted and passed in response to the growing nuisance of junk mail on mobile phones, specifies in part, that junk mail must be identified as such with the Japanese equivalent of "unsolicited advertisement" in the subject line.
The new filters will take advantage of this and automatically block such messages unless the user switches the filter off through an I-mode home page.
An existing service allowing users to include or exclude mail from 10 listed addresses or domains will also be expanded to help keep mail boxes spam free. NTT DoCoMo said it will expand the number of addresses or domains that can be blocked as part of the free service to 20.
Junk e-mail has become a major problem for Japanese mobile phone users, with those of NTT DoCoMo particularly badly affected. The large number of subscribers have made the carrier a better target for spammers than its two smaller rivals.
A second problem is now hitting mobile phone users, the so-called "one giri" call, also being targeted by the carrier.
The system involves a company calling vast numbers of mobile phones and hanging up after a single ring, just enough to get the caller's number registered in the phone memory as a missed call. The reaction of many users is to call back, even if they do not recognise the number, and they are then usually connected to a sex-related recording that switches to a higher tariff when the user presses a key.
NTT DoCoMo said it will begin offering several measures to help combat such calls, including a new ring tone that stays silent for the first ring and equipping phones with a counter to indicate the number of rings of each missed call. Users will also be able to register up to 20 numbers to be diverted to a recorded message advising the caller that the user is not accepting calls from that number.
One giri calls originate from computers, so the message is likely to mean little, but the measure is meant to hit the one-giri operators in the pocket because they will have to pay around ¥10 (5p) for each diverted call.