Post Office Protocol (POP) is a store-and-forward email service protocol

• Post Office Protocol: Version 2 - (IETF RFC 937, Obsoletes RFC 918) M. Butler, J. Postel, D. Chase, J. Goldberger, J.K. Reynolds, February 1985
• Post Office Protocol - Version 3 - (IETF RFC 1939, Obsoletes RFC 1725, Updated by RFC 1957 and RFC 2449) J. Myers, M. Rose, May 1996
• POP3 Extension Mechanism - (IETF RFC 2449, Updates RFC 1939) to be used by a server to express policy descisions taken by the server administrator, R. Gellens, C. Newman, L. Lundblade, November 1998