This could also be titled “Why you should use IMAP instead of POP3”
If your mobile device has a POP3 connection to the server, depending on your phone’s configuration settings, it is possible that messages are being deleted from the server. In this scenario, if messages are not being saved on the server, any other computers you are using for email will not be able to access your email.
To prevent messages from being deleted from the server with a POP3 connection to the server, you must specify in the phone’s configuration settings to save a copy of messages on the server. When messages are saved on the server, they will be accessible for other computers using POP3 or IMAP connections, for instance your desktop or laptop computers.
A better solution is to configure mobile devices using IMAP which synchronizes folders between the server and the phone. If a message is deleted from the phone, it is also deleted from the server. If a message is saved or moved to a folder on the phone, it will similarly be saved on the server.
The ultimate configuration solution is to configure all devices and computers using IMAP. This creates a properly synchronized email environment between all your devices and computers, displaying the same folders, showing the same read/unread messages and not having to delete messages multiple times.