Comparing Two Approaches to Remote Mailbox Access: IMAP vs. POP

by Terry Gray, Director, Networks & Distributed Computing at University of Washington

There are several different approaches to building a distributed
electronic mail infrastructure. For example: LAN-oriented, vendor
specific systems; single time-sharing machine solutions; and
Internet-oriented mailserver-based solutions. The principal options
in this last category are DMSP (Distributed Mail System Protocol),
POP (Post Office Protocol), and IMAP (Internet Message Access
Protocol). These protocols are more robust foundations for a
distributed e-mail system than vendor-specific systems requiring
gateways to Internet mail. Of these three, POP is the oldest and
consequently the best known. DMSP is largely limited to a single
application, PCMAIL, and is known primarily for its disconnected
(offline) operation capabilities. IMAP offers significant advantages
over POP. This gap is likely to widen as a result of the imminent
addition of disconnected operation extensions to IMAP.

With POP (Post Office Protocol), mail is delivered to a shared server,
and a personal computer user periodically connects to the server and
downloads all of the pending mail to the "client" machine. Thereafter,
all mail processing is local to the client machine. Think of POP as
providing a store-and-forward service, intended to move mail (on
demand) from an intermediate server (drop point) to a single
destination machine, usually a PC or Mac. Once delivered to the PC or
Mac, the messages are typically deleted from the POP server.

IMAP is a client-server mail protocol designed to permit
manipulation of remote mailboxes as if they were local. With IMAP,
mail is again delivered to a shared server, but the mail client
machine does not normally copy it all at once and then delete it from
the server. It's more of a client-server model, where the IMAP client
can ask the server for headers, or the bodies of specified messages,
or to search for messages meeting certain criteria. Messages in the
mail repository can be marked as deleted and subsequently
expunged, but they stay on the repository until the user takes such
action. Depending on the IMAP client implementation and the mail
architecture desired by a system manager, the user may save
messages directly on the client machine, or save them on the server,
or be given the choice of doing either.

While POP and IMAP both allow access to mail on a remote server
from a variety of different client platforms, they reflect two different
paradigms and styles of use. POP works best for people who use a
single client machine all the time; it is not well-suited for the goals of
accessing one's inbox of recent messages or saved-message folders
from different places and different machines at different times.

The strength of POP, other than its wide availability, is that it
minimizes use of server resources and connect time when used via
dialup. However, since IMAP is a functional superset of POP, it can
also be used in the "POP paradigm" of connecting to a mail server,
retrieving all the pending messages, and disconnecting. Thus, the
only advantage of the POP *protocol* over IMAP relates to software
availability and not functionality. As the amount of IMAP software is
growing rapidly, the historic prevalence of POP is of diminishing
importance when compared to the many advantages of IMAP.

Because IMAP can mimic all of the POP mail retrieval functions, it is
useful to distinguish the characteristics of the IMAP and POP
*paradigms*, as well as the protocols themselves. The paradigms
define what the user can do in each model; the protocol
characteristics relate to efficiency, performance, etc. Here are some of
the key similarities and differences between the two...

o POP and IMAP reflect two different paradigms:
  -POP = store-and-forward (usually to a single client).
  -IMAP = multiple client-server mailbox access.

o Characteristics common to both POP and IMAP:
  -Mail is delivered to a shared, "always up" mail server.
  -New mail accessible from a variety of client platform types.
  -New mail accessible from anywhere in network. -Offline mail processing
   possible, though neither designed for it.
  -Protocols are open; defined by Internet RFCs.
  -Freely available implementations (including source) available.
  -Clients available for PCs, Macs, and Unix.
  -Commercial implementations available.
  -Internet oriented; no SMTP mail gateways required.

o POP paradigm advantages:
  -Minimum use of connect time.
  -Minimum use of server resources.

o POP protocol advantages:
  -Simpler protocol; easier to implement.
  -More client software currently available.

o IMAP paradigm advantages:
  -Saved-message folders may be stored on server (as well as INBOX).
  -Allows access to INBOX (not just new mail) from multiple platforms.
  -Allows concurrent access to a shared mailbox from multiple
   platforms.
  -Allows concurrent access to mailboxes on multiple mailservers.
  -Offers improved offline mail handling.
  -Allows selective transfer of messages/parts to client (local Save).
  -Can also use POP paradigm, for minimum connect time and server resources.

o IMAP protocol advantages:
  -Suitable for accessing non-e-mail data; e.g., NetNews, documents.
  -Faster startup time, as only the headers are fetched initially.
  -Allows selective fetching of individual MIME message body parts.
  -Effective over low-speed links.
  -Ability to use server for searching.
  -Offline processing w/resynchronizing is a planned enhancement.

"Saved-message folders may be stored on server (as well as INBOX)"
allows "dataless" clients and/or nomadic users (e.g. student labs).

"Allows access to INBOX (not just new mail) from multiple platforms"
means that if you have a Mac in your office, and PC at home, and a
Unix machine in the lab, you can move freely among them and access
the same INBOX.

"Allows concurrent access to a shared mailbox from multiple
platforms." This capability is useful when multiple individuals are
processing messages coming into a common inbox. Changes in
mailbox state can be presented to all concurrently active clients via
IMAP.

"Allows concurrent access to multiple inboxes on multiple
mailservers." This is useful for people who have partitioned their
incoming mail streams, either via delivery filters, or by having
different accounts for different purposes.

IMAP "offers improved offline mail handling" compared to POP.
Unlike the DMSP protocol used in the PCMAIL program, neither POP
nor IMAP was designed with offline use as a primary goal. However,
POP is widely used for this, even though it is not particularly well-
suited for the task. POP requires you to either entrust all of your
mail to your client machine (which may be about to go thru an
airport xray machine), or to over-ride the normal POP server
behavior of deleting the mail on the server, and manually
resynchronizing the diverging mailbox states at a later time. IMAP
can do better: you can connect to the server, save to a local folder all
or selected messages, and disconnect. The advantages over POP are
that (1) the saved messages may be retained on the server, but
*marked* as deleted, so they can be distinguished later from
unselected or more recent messages, and expunged once it is clear
they won't be needed, and (2) the ability to save (download)
selectively --especially important when one has a 2MB audio
message in the mailbox and is reading mail via a low-bandwidth
connection from a machine that has no sound capability.

"Allows selective transfer of messages/parts to client (local Save)."
Especially when connecting to a mail server via low-bandwidth lines,
it is useful to be able to defer transferring messages that are not of
immediate interest until a more propitious time. Moreover, with
multimedia or multipart MIME messages, transferring selected parts
of a message in increasingly useful. Efficient processing of MIME
messages is one of the major advantages of IMAP over POP. MIME
stands for Multipurpose Internet Mail Extensions. It is a technique
for encoding arbitrary files as attachments to SMTP and RFC-822
compatible Internet mail messages. This is something that
proprietary, LAN-oriented, mail systems have had for some time,
and now is finally available for the Internet. It allows one to send
spreadsheets, word processing docs, images, and audio to 5 or 10
million of "your closest Internet friends". MIME has one particularly
nice capability: it allows inclusion of alternate representations. For
example, a plain-text version of a document, plus a fax or RTF
version. With IMAP, the receiving mail user agent gets to decide
which message parts to transfer and present to the user, falling back
to plain text if that's all it can do.

Even though MIME support is not yet pervasive, its importance and
impact on the IMAP/POP question should not be underestimated.
There is tremendous pent-up demand for this capability and it is
taking off fast. In fact, people are already using MIME for things that
have nothing to do with mail (e.g. encoding different representations
of technical documents, for campus-wide information systems.)
MIME is orthogonal to IMAP and POP, except that IMAP and MIME
are extremely complementary, and there are already IMAP clients
that understand MIME. (POP clients can and will be taught to
understand MIME, too, but the fact that POP copies all pending
messages at startup, and MIME messages can be very large, means
that POP users may need to become even more patient!)

"Can also use POP paradigm, for minimum connect time and server
resources." The POP paradigm is of interest in situations where the
only access to a mail server is via expensive dialup connections and
multi-platform access to one's inbox(es) is not needed. It is also
useful in environments where client machines are resource-rich and
servers are resource-poor. However, because IMAP is a superset of
POP functionality, IMAP can be used in "POP mode". That is, IMAP
clients can be designed to provide the option of transfering all
messages to the client and processing them locally (generally offline),
thus providing the same advantages POP has in terms of minimizing
connect time and use of server resources.

In summary, the fact that IMAP provides access to a persistent
remote mail store, and does not move all pending messages to the
client machine, offers more flexible access to that mail, and provides
significant performance advantages over POP in terms of start-up
time and access to large MIME messages. The option of accessing
remote saved-message folders and/or NetNews via IMAP (and from
the same mail user agent) provides additional architectural flexibility
in comparison to POP.

A reasonable conclusion is that the only advantage of POP over IMAP
is that there is currently more POP software available. However, this
is changing rapidly, and IMAP's functional advantages over POP are
nothing less than overwhelming.



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