Users of Apple's MobileMe have already discovered that the £59-per-year service is sometimes slow and unreliable, and they're now talking about another shortcoming that was intentional.

MobileMe allows users to synchronize email, calendar and contact information among various devices over the internet. Although the log-in process for MobileMe is encrypted, Apple does not encrypt data that users send from browsers through MobileMe. The lack of SSL (Secure Sockets Layer) or any other form of encryption means that if a MobileMe user is connected to the internet via a WiFi hotspot, someone else connected to the same hotspot could relatively easily see all the data that the MobileMe user sends.

"Seems like a pretty major omission for a service that's specifically aimed at roaming users," wrote a person called ShepUK on the Macrumors forum. He called the lack of SSL encryption a deal breaker for him.

Free web mail offerings from Yahoo and Microsoft also don't encrypt data that users send, though Google's free Gmail does offer an SSL encryption option. However, MobileMe has more features for transferring data over the web than do those services.

Because customers must pay to use MobileMe, some people think it ought to have this basic security feature. "MobileMe is suppose to be Microsoft Exchange for the rest of us. But Microsoft Exchange does things in a secure manner," wrote a commenter using the name James Katt in a blog postM about the SSL issue. "As it is, if you run a business using your Mac, then you cannot use MobileMe because it transmits data insecurely."

Still, while security professionals say Apple should offer an SSL option, they don't suggest that this is a major problem. "I wouldn't say that it's a critical issue or something that's a reason not to use the service, but it's definitely something that should be addressed," said Noam Rathaus, CTO of Beyond Security, a company that offers products for discovering security issues in servers, computers and networks. He briefly examined the way MobileMe works and said it does appear that transmissions aren't encrypted.

Apple has not replied to a request for comment.

The setup could open the door to potentially serious situations in which an onlooker could learn passwords used by a MobileMe customer if the user requested a lost or forgotten password to be sent via email.

If MobileMe users are concerned about the security of information, they may want to use another mail mechanism to send sensitive messages, said Wolfgang Kandek, chief technical officer for Qualys, a provider of vulnerability-management and policy-compliance products. VPN (virtual private network) connections could protect users, but some enterprises set up VPNs to protect access only to corporate servers and not to Internet sites. In that case, a VPN wouldn't protect a user of MobileMe, he said.