What is POP3?
POP (Post Office Protocol) is used to provide simple remote access to mailboxes. POP3 is the latest version of POP. With POP3, once mail clients connect and download their messages, the messages are typically removed from the server and stored on the client's local machine.
This means that you have to ensure that the all the machines are backed up in order to prevent you losing any information in email messages. To resolve this problem, you could use the IMAP4 or Web Mail clients which allow messages to be manipulated on the server.