IF you are using Outlook to download your email to your desktop PC, Outlook is likely “deleting” mail from your email server when it downloads it to your PC.
There is a setting in Outlook to “Leave Messages on Server”.
For MS Outlook 2003:
Outlook > Tools > Email accounts > View or Change existing > next.
Highlight the email account > Change > More Settings.
Advanced Tab > Leave a Copy Messages on Server
Check that box.
- You will need to perform this edit for each Outlook email account you have.
Other versions of Outlook have a similar setting although it might located in a slightly different location in the :advanced settings”