Subject: UIDL

From: [email-masked]
Date: Tue, 19 Jun 2001 15:50:20 -0700

Hi Mark,

I understand that in pop3 each message has unique identification UIDL
which persists across sessions. What is the use of UIDL ? can this UIDL
be used to fetch/change status of messages ? What I mean is can this
number be used in place of message number to make modifications to
the message in the mailbox ?

Thanks,
Manjula

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From: Mark Crispin <[email-masked]>
Date: Tue, 19 Jun 2001 15:50:20 -0700 (PDT)

On Tue, 19 Jun 2001 15:50:20 -0700, [email-masked] wrote:
> which persists across sessions. What is the use of UIDL ?

UIDL gives you the mapping between unique identifiers and message numbers in
that session.  You can use this map to locate unique identifiers that you
recorded in an earlier session, and get the current POP3 message number.

> can this UIDL
> be used to fetch/change status of messages ?

No.

You must use the message number to fetch messages.

There is no such thing as "status of a message" in POP3.

> What I mean is can this

No.  There is no such capability in POP3.

You should use IMAP instead of POP3.

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From: Julian Hall <[email-masked]>
Date: Wed, 20 Jun 2001 11:34:30 +0100

Just to clarify this:

the only real purpose of UIDL in POP3 is to allow you to easily tell which
messages in the mailbox you have already seen (by recording a local copy of
the id of each message you have seen).  This allows you to write a mail
client that leaves its mail in the POP3 folder and doesn't get duplicated
mail in the inbox.  UIDL is an optional feature of POP3 so not all servers
have to support it.  For servers that do not support UIDL there are various
other extensions that provide for leaving messages on the server; some
servers have a 'LAST' command that retrieves the ID of the last message
that was seen (you can generally assume that if a server has such a
command, the messages are kept in order of arrival in the mailbox). Another
alternative is to use the 'TOP' command to retrieve the headers and compare
the message-id fields (although this is a particularly unreliable method).