POP3 (Post Office Protocol version 3) is a protocol for receiving E-mails. It is a method of accessing E-mail messages via an E-mail client such as Outlook or your mobile. When an E-mail is sent to your address, it is stored on the mail server until it is downloaded via your client using POP3. After its download, the E-mail is removed from the server altogether.
If you to set up an E-mail with POP3 account on a mobile phone, the phone would download all E-mails, removing them from the server. If you would check 1&1 WebMail on the home computer for the same E-mail account, nothing would be displayed as the messages have already been downloaded by the mobile phone.