How does "leave a copy" work?

[Jack L.]

Hello group.

Outlook and Outlook Express both have a "leave a copy" feature, and I'm
curious how they know which emails to download from the server, and which
one not to download. Do they have a database somewhere in the disk for which
they use to compare with the list of emails in the mail server or something?
More precisely, I'm wondering why Outlook/OE sometimes loses this
information somehow and redownloads everything again, and then I have to
delete them since I already have them.

 

[Brian Tillman]

Outlook and Outlook Express both have a "leave a copy" feature, and
I'm curious how they know which emails to download from the server,
and which one not to download. Do they have a database somewhere in
the disk for which they use to compare with the list of emails in the
mail server or something? More precisely, I'm wondering why
Outlook/OE sometimes loses this information somehow and redownloads
everything again, and then I have to delete them since I already have
them.

Any POP client has this ability. What the POP client does is issue the UIDL
command to the POP server, which causes the POP server to assign a value to
each of the messages in the server Inbox. When the POP client downloads
messages, it keeps track of the UID values for those messages so that the
next time it asks for messages, it will skip the ones for which it has the
UID already. If the POP client somehow forgets these numbers, it will
believe the messages new again and redownload them.

Some POP clients keep the UIDs in a separate file. Frankly, I don't know
where OE keeps them. Outlook, for some versions, keeps them in the mail
profile in the registry. Other versions keep them in the PST in a hidden
message, with the message being tied somehow to the mail profile. For any
version, Outlook's profile gets damaged in specific ways or you create a new
mail profile, Outlook will forget the UIDs and begin the process afresh,
redownloading old messages. The usual fix is to create a new mail profile.