The POP email protocol makes a copy of each email onto each device connected to the server. This means that if an email is deleted from the server, a copy remains on each device. Inversely, if a copy is deleted off a device, it remains on the server or other devices.
Instead of copying emails to different devices, the emails are synchronized with each device; therefore, any changes made on a device are synchronized with the server and other devices. In other words, a copy of each email received is stored in each location, such as your home and office computers, along with the email server. If you delete an email from one computer or the email server, it is deleted from all other locations.