Mobile Email on iOS devices

Hi All,

I have a client that has this weird thing happening on his iOS device (iPhone4s), the current setup of their email system is everything will be moved in .pst, so if the user received an email to his/her mobile device, and he downloaded all email from his device going to his/her pst file, the email on mobile will be empty. But in his case, there's some email that remains in his mobile email, although it is already downloaded to his/her pst file.

Any workaround here? or any fix about this?

Thank you!