Mac Forum / Applications / Eudora / March 2008

You could try setting up other email clients
to do exactly the same thing (including "LMOS for 7 days"),
and if they, too, get the same duplicates,
then that leaves Eudora out of the picture,
and your ISP as the only common element.

The trick which allows clients to recognize previously seen messages,
without actually downloading their entire content,
is for the POP server to "fingerprint" each message,
in the form of a short string called a "Unique ID."

The mail client first downloads all the "fingerprints" at once,
recognizes any that match previously downloaded messages,
then downloads only those messages having new "fingerprints."

If the server is consistent in making its fingerprints,
and the client is reliable in remembering and matching them,
then the system works, and each message is downloaded only once,
even if left on the server.

The client retains fingerprints only so long
as the server keeps sending a match, since fingerprints
for messages no longer on the server no longer need to be saved,
but if the server ever suffers from "temporary amnesia" itself,
and falsely says "gee, your mailbox is empty,"
causing the client to to say
"great, then I can throw away all my stale fingerprints,"
then that, too, could deceive a client into re-downloading.

Technically, this "fingerprinting" capabilitity is optional
for POP servers, but since it's the only reliable way
to completely solve all issues about messages independently
left on the server, it is probably never omitted nowadays.

A client could also be caused to have "amnesia" about previous
downloading of messages if it lost its "notes" from previous sessions,
but when a whole family who share mailboxes are all simultaneously
unable to recognize previously seen messages, it seems more likely
to be the server becoming inconsistent in making its fingerprints.

If the ISP is in fact having problems in that department,
they should be hearing from many other people about it,
and should be willing to let you know if that's happening.

We had it happen just once, when replacing our server software,
at which point all the new-style "unique IDS" of messages left
on our server did not match the old fingerprints saved by clients,
causing them to download, just once, every message still on our server,
after which the new IDs remained consistent, going forward in time.

--

Actually, you are better to have only a single computer delete the
messages, rather than both. If both delete messages, and if you ever
don't check for new mail on one of the machines for more than a week,
then you run the risk of some messages being on one machine but not the
other and vice versa. In other words, no single machine will have all
the messages on it.

It's usually better to make one machine the master. This will be the one
that deletes messages after a week. Since it will be the only one to
delete messages, it will only delete them after it has downloaded them,
and therefore will have a copy of all incoming mail messages.

The other machine should be set to leave messages on the server and
never delete them.

Cheers

David

There's not enough information here to do more than guess, but my
*guess* is that your ISP may have changed server software.

The best guess I have is that your ISP had been running a POP3 server
that maintained a "Status" header on messages, which Eudora can use to
support the sort of setup you describe. Historically, it was the *only*
way Eudora tracked which messages had already been seen. Even if you
have Eudora set to the better approach of tracking message UID's (set in
the "Ports & Protocols" settings pane) any instance of Eudora will still
have no way to tell that some other client has downloaded a message if
the server does not support the Status header. There are valid arguments
against the Status header technically, since it requires the POP3 server
to modify messages and has never been formally standardized. As far as I
know, the only mailstore servers of any sort that support the Status
header are older ones like qpopper and the UW POP3 server. Many mail
providers these days need more sophisticated mail servers and have been
moving on to modern POP3 implementations like Courier and Dovecot that
do not offer the non-standard Status header.

The subtext for the abandonment of the Status header is that it offers a
limited and poorly-defined subset of the functionality for POP3 and
direct mbox clients which is specified in an extensible and clear way in
the IMAP protocol. There are intrinsic problems with using POP3 for
multiple-client access and sharing of message state, while IMAP is
designed for that. Most modern POP3 servers are *also* IMAP servers, so
you should ask your ISP if they've recently changed server software and
if that means you can use IMAP instead of POP3. Eudora is not a great
IMAP client, but it does basically work.