I have a Nokia 6610, and I'm having similar problems to those experienced by Trung.
There are some mails I cannot open; i get a "Response Unknown" message and are returned to the main index. In addition, i only have access to my main Inbox.. if i use the 'Change Folder' link, i am taken to a page where only Inbox is listed, in addition to a 'Submit'-link. If i click the latter i get this error message: "Warning: Couldn't open stream {mail.messagingengine.com:143}INBOX in/var/www/wap2/index.phpon line20 ERROR: Can't connect:"
Other than that I'm pretty pleased to be able to check (almost all) my mails on my phone
nils