Take outlook21 as an example:
1. Open Outlook;
2. Click the file tab;
3. Click Account Settings;
4. Double-click the email account;
5. click other settings;
6. Click the Advanced tab and check to keep a copy of the mail on the server. If you want to keep it for a long time, click Uncheck xx days to delete the copy of the mail on the server, and then click OK.