POP refers to the protocol used by e-mail software, such as Eudora or Outlook Express, to retrieve electronic mail from a mail server. The protocol used by mail clients to retrieve messages from a mail server. This includes POP1, POP2, and POP3, the number denoting the different version number of the protocol. POP3 is the most common e-mail standard. POP is the protocol used by mail clients to retrieve messages from a mail server. See Also: E-mail, Protocol