A simple tutorial that explains you the procedure to leave a copy of email messages on server.
Check here the procedure for setting outlook to leave a copy of emails on the server. This will prevent your mails from being deleted once after reading.
Step 1:Open Microsoft Outlook. Under Tools Menu, choose Email Accounts option.
Step 2: Click on More Settings.
Step 3: Click the Advanced tab. Enable the "Leave a Copy of Messages On Server" check box to prevent your email from being deleted after reading and click on Ok.
Step 4: Now, your mails will not be deleted from the webmail after reading.
Note: All the above given steps are for Outlook application to leave a copy of emails on the server. The procedure may vary for other applications.