By Andrea Orr

PALO ALTO, Calif. (Reuters) - The greeting "hello" in on e-mail headline used to signal a welcome note from a friend. Increasingly, it is from a stranger you probably don't want your children to be talking to.

Last week, a click on a "HELLO" greeting yielded a note from someone named Lisa who said she had just turned 18 and promised that the sex acts she performed on her Web site were "TOTALLY LEGAL!"

Legal as she claimed to be, Lisa was not all that discreet in describing the X-rated contents of her Web site, and as anyone who uses e-mail knows, the details of her mass mailing the least bit unusual.

While junk e-mail, or spam has been a nuisance for years, it has by almost all accounts become much more pervasive -- and offensive -- in recent months. Increasingly, the offensive material is arriving with friendly greetings -- partly to get readers to click, but also to avoid technology that heads off explicit sexual content before it gets to users' mailboxes.

"There has been a significant increase in spam over the past several months, close to double-digit growth," said Dan Birchall, director of the Spamcon Foundation, one of several groups that assists consumers in blocking unsolicited e-mail.

There are multiple reasons for this spike, but it is essentially a matter of the rewards outweighing any perceived risks. Bulk e-mail providers will send a company's ad to one million e-mail addresses for as little as $200, and consumers have to date been mildly annoyed but not really outraged. Regulators, meanwhile, have gone after only the most egregious forms of e-mail deception.

But with the recent increase in spam, there has come a lot more sexually explicit material moving through e-mail, and more sly practices for concealing the true subject matter. Now, Internet users are increasingly finding this deluge of spam unacceptable.

INCEST, VIAGRA AND BRITNEY

"My oldest son just turned nine and he is starting to understand the concepts of romance and sex and attraction," explained Barbara Grady, a mother of two young boys who lives in Albany, Calif., works from home, and has accounts with both AOL Time Warner Inc's AOL.N America Online and EarthLink Inc ELNK.O . Grady said the content of the unsolicited mail she receives on both accounts has become too objectionable for her to allow her sons to use the Internet unsupervised.

Her view seems hardly extreme considering the adult-related e-mail that pours daily into consumer accounts. Some of the spam mails that have circulated recently have promised "guaranteed penis enlargement," "get it up and keep it up with Viagra," "thousands of your favorite nude celebrities, and "teen lesbians, teen amateurs and MORE."

And those are just the most printable ones.

More troubling to parents are all the spam mails that lure unsuspecting surfers with vague or completely misleading headlines. One mailing labeled "Innocent Sister" led into a streaming-video Web site called "Incest at its Finest." Other ads start the header message with the prefix like "Re:" or "Fwd:" to make it look like it is a response to an ongoing conversation initiated by the recipient -- rather than an intrusive ad from a shady operation.

A piece of mail labeled "My New Phone Number," opened onto a link featuring "what Britney does when nobody is home." Another ad for a Viagra site appeared to be directly targeting children with the teaser "Funny Cartoons."

"I do feel it is ridiculous that not more has been done," said Grady, who says she is skeptical when some of world's biggest technology companies say they are stumped by the problem. "Technologically, I'm sure that the ISPs could do more."

Internet service providers say their struggle to combat spam is like a game of cat and mouse in which new solutions are always needed for increasingly sophisticated spamming techniques.

Just this week, the U.S. government stepped in to shut down e-mail scam that promised free video-game consoles but instead delivered a connection to a pornographic Web site that charged $3.99 per minute.

FILTERS NOT A POPULAR SOLUTION

Mainstream Internet companies try to distance themselves from such practices and to highlight steps they've taken to safeguard users.

"Yahoo has taken aggressive steps to combat spam," said Yahoo! Inc YHOO.O spokeswoman Mary Osako, citing the company's SpamGuard service that automatically routes bulk e-mail to a separate folder.

Microsoft Corp MSFT.O offers similar solutions at its free HotMail e-mail service, including filters that let users block e-mail that contain certain keywords, or that come from unknown addresses.

Still, it is worth noting that many spammers are using HotMail and Yahoo accounts to send mass advertisements about pornographic Web sites. And some critics suggest that Internet service providers need to do more to attack unsolicited e-mail, especially the sexually explicit kind, at its source.

"Spam is certainly a problem that continues to grow and it keeps pace with the different solutions," said Parul Shah, a product manager for MSN. She said the relatively new practice of using misleading headlines lets more mail slip through filters. Some U.S. states have laws against misleading headlines, although they are not always enforced.

But many Internet users say even the best filter is not an adequate solution, since it severely curtails their ability to use the Internet.

Grady, for instance, is a freelance journalist who depends on unsolicited tips she receives through e-mails. Others like Joel Miller, a father of a 10 year-old and a 14-year old in Chevy Chase, Md., says his children are more equipped at undoing filters than he is at setting them.

Besides, people insist it is not unreasonable to want to be reachable to any long lost friend who might surface with an old-fashioned "Hello" greeting. They say blocking mail from all unfamiliar addresses is a bad solution that puts too much of the burden of blocking spam on them.

"I' sure, technology-wise, the Internet service providers could do more," said Miller. "I am not sure why they don't."