UK based company Synchronica gives corporations the ability to use enterprise email systems from Microsoft, Lotus, and Sun with the new iPhone.

From the MacWorld article:

The software talks directly with the iPhone’s IMAP/SMTP mail client, and then connects securely to corporate e-mail servers: Microsoft Exchange or Lotus Domino, as well as Sun Java Enterprise System. In the case of Exchange, Mobile Gateway uses Microsoft’s secure Outlook Web Access to retrieve e-mail from the Exchange server. The e-mail is then sent to the iPhone client via IMAP.

[...]

“If I delete a spam [message] on my iPhone, it is deleted in Exchange,” Brinkshulte says. “If I send an e-mail from the iPhone and then come back to office, it is shown in the ‘sent-items’ folder of my Outlook/Exchange [screen]. There’s no need of ‘copy to self’ workarounds.”