Some Observations on Implementations of the Post Office Protocol (POP3)
Real-world variations in how mail servers and clients handle POP3.
Nelson, R., "Some Observations on Implementations of the Post Office Protocol (POP3)", RFC 1957, DOI 10.17487/RFC1957, June 1996, <https://www.rfc-editor.org/info/rfc1957>.
@misc{rfc1957,
series = {Request for Comments},
number = 1957,
howpublished = {RFC 1957},
publisher = {RFC Editor},
doi = {10.17487/RFC1957},
url = {https://www.rfc-editor.org/info/rfc1957},
author = {R. Nelson},
title = {{Some Observations on Implementations of the Post Office Protocol (POP3)}},
pagetotal = 2,
year = 1996,
month = jun,
} RFC text
Contents
Network Working Group R. Nelson
Request for Comments: 1957 Crynwr Software
Updates: 1939 June 1996
Category: Informational
Some Observations on Implementations
of the Post Office Protocol (POP3)
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Observations
Sometimes an implementation is mistaken for a standard. POP3 servers
and clients are no exception. The widely-used UCB POP3 server,
popper, which has been further developed by Qualcomm, always has
additional information following the status indicator. So, the
status indicator always has a space following it. Two POP3 clients
have been observed to expect that space, and fail when it has not
been found. The RFC does not require the space, hence this memo.
These clients are the freely copyable Unix "popclient" and the
proprietary "netApp Systems Internet Series". The authors of both of
these have been contacted, and new releases will not expect the
space, but old versions should be supported.
In addition, two popular clients require optional parts of the RFC.
Netscape requires UIDL, and Eudora requires TOP.
The optional APOP authentication command has not achieved wide
penetration yet. Newer versions of the Qualcomm POP server implement
it. Known client implementations of APOP include GNU Emacs VM client
and Eudora Lite and Eudora Pro.
Security Considerations
Security issues are not discussed in this memo.
References
[1] Myers, J., and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
Author's Address
Russell Nelson
Crynwr Software
521 Pleasant Valley Rd.
Potsdam, NY 13676
Phone: +1.315.268.1925
FAX: +1.315.268.9201
EMail: nelson@crynwr.com
AI ELI5
On this page
§1Implementation Variations
RFC 1957 documents observations on POP3 (Post Office Protocol version 3) implementations that diverge from the standard. Sometimes an implementation becomes so widely used that it becomes a de facto standard, even when it does not comply with the published specification.
This memo serves as practical guidance for implementers who must maintain compatibility with popular but non-compliant software.
This RFC notes that some mail programs do things differently from the official rules, and other programs have learned to expect those differences.
§2Status Indicator Format
The widely-used UCB POP3 server implementation, popper, always places a space following the status indicator in responses. Although the POP3 standard does not require this space, two popular mail clients—the Unix utility popclient and the proprietary netApp Systems Internet Series—expect it and fail without it.
The memo documents this mismatch so that implementers are aware of the requirement for backward compatibility, even though the RFC does not mandate it. Authors of both clients were contacted, and newer versions were released to handle both formats.
Some popular mail client programs expect an extra space in the server's response, even though the rules do not require it. New versions were fixed, but old ones still need that space.
§3Optional Features in Wide Use
The POP3 specification defines several features as optional, but popular clients have created de facto requirements. Netscape requires the UIDL (Unique Identifier Listing) command, while Eudora requires TOP (read message header without fetching the full message).
The APOP (Authenticated Post Office Protocol) command provides better security than the standard login method, but adoption remained low as of 1996. Newer versions of the Qualcomm POP server and clients like GNU Emacs VM and Eudora added APOP support.
Some mail programs depend on optional features even though they are not required. As time went on, more servers and clients added support for the safer APOP login method.