If the option to Leave a copy of the message on the server is missing in Outlook, it may be due to the type of email account you use. Of the most common types of email accounts, only POP3 accounts download the email to your computer.

If you have an IMAP, or HTTP (Gmail, Outlook.com etc.) account, mail is not generally stored on your computer. All email remains on the mail server until you delete it.

To enable (or disable) the Leave a copy of the message on the server option, follow these steps:

Outlook 2010 - 2013
 

1. Open Outlook.
2. Click the Filetab > Account settings.
3. Highlight your current POP3 account and click Change.
   Note:The window that opens will tell you what type of email account you have. (Look under Type.)
4. Choose More settingsand click on Advanced.
5. You should see a check box titled: Leave a copy of messages on the server.
6. Unchecking the box will disable the function to Leave a copy of the message on the server.

Mac Mail

1. Click “Mail” on the menu bar at the top of the screen
2. Click “Preferences…” then Click “Accounts” on the window that opens
3. Select your account on the left-hand side
4. Tick the box labelled “Remove Copy from server after retrieving a message:” and select a timeframe from the box below 

Note: Enabling Leave a copy of the message on the server will allow you to access your email from multiple computers, however if you primarily use email on the same mail client it will download deleted emails each time you log in.