Spam accounts for an estimated 10 percent of all e-mail worldwide, clogging recipients' in boxes and slowing routing computers.
Most spammers conceal their own e-mail addresses, making it hard for computer users to retaliate.
The new version of Sendmail, however, will verify return addresses by looking them up on a central Internet registry before relaying the e-mail. Sendmail author Eric Allman said the new version also has the ability to reject mail from known junk mail originators by checking a widely circulated list of spammers, called the Realtime Blackhole List. Allman said he would continue to offer Sendmail as freeware, meaning it is available for free on the Internet.