Push e-mail provider Visto says it will be able to send enterprise e-mail to iPhones starting later this year.

Microsoft Exchange and Lotus Domino e-mail users will be able to send and receive messages on the iPhone, either as an individual user or on a corporatewide basis. Apple's much-hyped iPhone goes on sale on Friday.

Individual users will download an application to their PCs. As e-mail messages arrive, that application transfers them from the PC to Visto's network operation center and then on to the user's iPhone. Users will automatically receive the e-mail in the standard client software that comes with the iPhone, said Haniff Somani, vice president and chief architect at Visto.

Users must leave the PC on for the e-mail messages to reach their iPhone. To set up the service, a user will enter Visto's server address into the IMAP application on the phone, he said. IMAP is a standard messaging protocol.

An enterprise could also buy server software that would push e-mail out to all the iPhone users in the company in a process that works in a similar way to the PC application.

Early reviewers of the iPhone have written that users will be able to receive Exchange e-mails on the phone, but Somani pointed out some reasons that might not be a popular idea at enterprises. In order to push Exchange e-mail to the iPhone directly, IT administrators would have to open a hole in their firewall that allows the IMAP data to flow back and forth, he said. That can be dangerous because it opens a door to potentially malicious activities.

Analysts agreed. "IMAP is not something they want to open up," said Ken Dulaney, an analyst at Gartner. "Most enterprises, when they hear that, say 'There's no way I'm doing that.'"

In addition, the direct IMAP services usually have poor performance, he said.

With Visto's service, the enterprise doesn't have to open the IMAP port because the Visto server resides behind the firewall, Somani said.

Ideally, Apple would license Microsoft's ActiveSync in order to support Exchange e-mail on the phones in a more robust and secure way, Dulaney said. That's how Nokia, for example, supports push e-mail to certain devices, he said.

He suggests that enterprises may be wary of the Visto offering because Visto is associated with mobile operator-based services and enterprises don't always trust operators with such services. In this case, Visto is hosting the network operations center itself but must still work with AT&T, which has said it will enable IMAP in its network for the service, Visto said.

Starting late in the third quarter, Visto plans to offer a beta version of the service that will be free to use for 60 days. iPhone users can register their interest in the beta on Visto's site. Visto isn't yet saying how much it will charge for service.

Visto typically requires users to download a small application on their phones that improves the speed with which they get mail and consumes less data than IMAP, Somani said. However, Apple is only allowing third-party applications for the iPhone that are browser-based, so companies like Visto can't write software that can be loaded onto the phones.

In April, a source at AT&T said the operator was planning to support the iPhone from a billing and customer support perspective for enterprise customers. However, an FAQ recently posted to AT&T's Web site says the iPhone will only be available to consumer accounts.