Mac Mail POP3 downloads emails from a server to your PC, and then deletes them from the server after 7 days by default. To change this setting:
1. On you Mac Mail app, click on Mail and select on Preferences….
2. Click on your POP account. You will see the “Remove copy from server after retrieving a message” option, where you can change to delete the emails immediately after download, after one day, one week or one month. If you want to leave emails in the server without deleting them, uncheck this option.