I have tried this and can confirm that it is a problem.

Yet again, MS, touting security is job one, has proven that they are all about lip service and not much else.

From http://news.com.com/2100-7355-5107904.html?tag=nefd_hed

Mail server flaw opens Exchange to spam
Last modified: November 14, 2003, 3:47 PM PST
By Robert Lemos
Staff Writer, CNET News.com

Administrators of e-mail systems based on Microsoft's Exchange might have spammers using their servers to send unsolicited bulk e-mail under their noses, a consultant warned this week.

Aaron Greenspan, a Harvard University junior and president of consulting company Think Computer, published a white paper Thursday detailing the problem, discovered when a client's server was found to be sending spam. Greenspan's research concluded that Exchange 5.5 and 2000 can be used by spammers to send anonymous e-mail. He says even though software Microsoft provides on its site certifies that the server is secure, it's not.

"If the guest account is enabled (on Exchange 5.5 and 2000), even if your login fails, you can send mail, because the guest account is there as a catchall," he said. "Even if you think you've done everything (to secure the server), you are still open to spammers."

The guest account is a way for administrators to let visitors use a mail server anonymously, but because of security issues, the feature is generally not enabled. Exchange servers that had been infected by the Code Red worm and subsequently cleaned will still have the guest account enabled, Greenspan said.

There are dozens of messages--with subject lines such as "Open relay problem" and "We are sending spam?"--on Microsoft's Exchange Administration newsgroup, sent by information system managers who haven't been able to staunch the flow of spam from their servers.

Microsoft, however, said the problem is relatively minor and that the company hasn't had many complaints.

"This particular method of sending spam relies on specifically configured servers or is leveraging weaknesses in the protocol itself," the software giant said in a statement issued in response to questions from CNET News.com. "The fact is that Microsoft has not received a lot of calls from customers that have experienced problems detailed by Think Computer."

Moreover, the company said the issue doesn't affect the latest version of the software, Exchange Server 2003.

Greenspan, however, argued that the problem has accounted for a large amount of unsolicited e-mail. He estimates that at least 100,000 messages spammers in China sent went through his client's server before he stopped the problem. He added that the issue is causing headaches for Exchange administrators.

"It is really inexcusable for a company that claims security is its top priority," he said.