[TAG] IMAP4 vs POP3

[Mike Orr]

On 11/21/05, Martin J Hooper <martinjh at blueyonder.co.uk> wrote:
> I'm just curious as to what the difference between IMAP4 and POP3 mail is.

Before POP and IMAP, the mail reader had to run on the server
containing the mail.  Either the user's workstation was the server
(bad if it broke or the user moved), or the user telnetted to the
server and ran a text program (bad if you like graphical programs;
difficult to attach files), or ran a remote graphical program on the
server (good for X-windows; bad for Windws/Macintosh).  Or you could
NFS-mount your mailbox (notoriously unreliable; easy to lose mail if
two programs use different locking strategies or have substandard
locking code).

POP was invented to support client mail readers: programs like
Netscape Mail and Eudora that downloaded messages from a central
server.  This was necessary on Windows desktops, where both mail
daemons and telnet are foreign.  It also brought less-technical users
to ISPs, which responded by offering POP servers and phasing out shell
accounts.  Some of these mail readers were ported to Linux, although
*they* seemed foreign there.  Having POP-reading and SMTP-sending code
in each mail reader seemed like the Wrong Way.  Fetchmail was invented
to provide a general way to download mail, and the general evolution
of sendmail alternatives kept an eye on desktop needs (simple
"smarthost" configuration).  But the code remained in the client
programs as an option.

IMAP was invented after POP but was present throughout the
transformation of ISPs and Linux mentioned above.  IMAP solved the
portability problem: what if I want to read mail from several
locations or open a message with a different mail reader?  IMAP has a
more sophistocated query language: it can distinguish between seen and
unseen messages, apply arbitrary flags to the messages, and do sorting
and searching on the server.  You *can* download messages in POP
without deleting them from the server, but if you go to another
computer you'll see the same messages again as if they were new, so
it's up to you to manage this in your head.  IMAP also has a
"subscription" feature which allows mail readers to function as USENET
news readers too.  IMAP and 'pine' (a mail reader) were invented by
the University of Washington to make email and news accessible
campus-wide by people not familiar with Unix.  They also distributed
campus announcements through local newsgroups.  It's surprising now
that thousands of Windows users would run NCSA Telnet to read their
mail, but webmail did not exist in the early 90s. The alternative was
proprietary email systems (e.g., on Novell NetWare), which did not
scale to heterogenous environments (the departments were largely
autonomous), or to logging in from home via modem.

> I was wondering about this as I noticed that my ISP does IMAP4

IMAP4 and POP3 are the current versions of these protocols.

> I really know nothing about IMAP but I gather that you can read your
> IMAP mail from two or more computers without having to download your
> mail at all..

That's exactly what IMAP does.  Since IMAP can also do everything that
POP does, I wouldn't use POP unless you have a program or server that
doesn't support IMAP.

> Here's what I would like to do...  At the moment most of my general
> email goes to martinjh at blueyonder.co.uk Normally that gets downloaded
> using Thunderbird to my Windows PC.
>
> When I am using Linux I would like to be able to read that e-mail
> without downloading it to the Linux side of things.  I wouldn't mind
> being able to use IMAP on both sides if it would help.
>
> What would be nice if I decided to use IMAP instead is having filtering
> work on both Linux and Windows too ;)

If you're using Thunderbird, it's a simple reconfiguration.  Go to
"Edit : Account Settings".  I would add an IMAP account and get
comfortable with it before deleting my POP account.  But first disable
all the automatic checking and deleting in your POP account ("Server
Settings").  You can also read up on fetchmail and run it a few times
to see what's going on at a lower level.

--
Mike Orr <sluggoster at gmail.com>
(mso at oz.net address is broken)