[Gllug] Email Folders

[Mike Brodbelt]

On Mon, 2003-06-09 at 22:13, Simon Faulkner wrote:
> Please may I ask a question which may demonstrate a total lack of email
> understanding?
> 
> I am used to receiving email in Outlook using POP3 from my linux box which
> runs Postfix (stock RH8)
> The rules wizard does a good job of allocating all the email to their
> correct folders in Outlook.
> 
> But, I would like to use IMAP so I can access my mail via Mozilla Mail,
> Outlook and Squirrel Mail as required.
> 
> When email comes in via Exim (or Postfix) I would like to use procmail to
> allocate it to different folders but does this then mean that it can't be
> read by POP3 anymore?
> 
> What folders does procmail use?  Just standard folders in your ~ directory?

Mail is received by your MTA (PostFix, Exim, sendmail) and then
delivered, usually to a local spool area on the disk. When you are using
POP3, the normal route is to have a POP3 server that looks at that mail
spool on the disk, and serves messages from it over POP3.

If you want to use IMAP, that implies that you need more than a simple
spool area - this tends to be referred to as a mailstore. You will
usually have a hierarchy of folders, which are accessible via IMAP.
Assuming you can find a POP3 server that understands the mailstore used
by your IMAP server, you can typically retrieve messages from your IMAP
inbox with POP3.

If you do this, you should probably set the POP client to leave messages
on the server, and not delete them. In general, you're better off making
a choice of IMAP or POP, not trying to use both.

The big picture is that you need several components for a mail system:-

MTA - this sends and receives mail via SMTP, and usually also performs
local delivery.

Mailstore - this is where the mail ends up.

IMAP/POP server - this mediates between the mailstore and mail clients,
providing the messages with a common protocol like IMAP or POP3.

You have several choices of system. For IMAP servers, you basically can
choose between Courier, UW, and Cyrus. Courier used maildir as the
mailstore format, UW can use mbox, maildir, or several others, Cyrus
uses its own format, optimised for IMAP use - it is also the only format
that provides a single instance store.

The format of the mailstore may be dictated by your choice of IMAP
server, and may also dictate your choice of MTA to some degree. Cyrus
requires that delivery be carried out via LMTP, so you should use a MTA
that can do LMTP, for Courier systems, you want a MTA that can be easily
configured to handle maildir. There are many options.

Procmail filters mail into folders, and understands several mailstore
formats. It is usually invoked by the MTA before local delivery. It is
not easily usable with a Cyrus system, as it does not understand the
Cyrus mailstore format.

If you are receiving mail from an ISP who provides only POP3 service,
you'll need to bolt fetchmail onto the front end to feed your MTA via
SMTP.

FWIW, I run sendmail/cyrus based mail systems. Spam filtering is done by
passing all mail through spamassassin via sendmail's libmilter
interface, and message filtering is done on the server with Cyrus' built
in sieve scripting language. People will have religions wars about MTA's
for ever, but among most people running serious IMAP systems, Cyrus is
the preferred option, as it offers several features found nowhere else.
For a personal system however, it may be overkill.

HTH,

Mike.


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