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I've moved over all emails from 10 accounts on one server to another. I use IMAP so this won't be an issue for myself, but someone else using these email accounts uses pop3. When I change the settings on their computer, Thunderbird will (I assume) download all the messages of the server, even though they are on their computer already.

Is there a way to force Thunderbird to only download emails from (for example) today as well as leaving messages on the server? We already have the "leave messages on the server" checked, but we don't want to download 10,000+ emails that were sent before today.

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The file "popstate.dat" file is what keeps track of which emails have already been downloaded from the POP server.

An example on my Windows XP Home PC is:

C:\Documents and Settings\Alan\Application Data\Thunderbird\Profiles\qn5qd3de.default\Mail\\popstate.dat

# POP3 State File
# This is a generated file!  Do not edit.

* AWehmann
k AHfVimIAAFFZTwJOOAAQHz0XYU8 1325822549
k ALnPjkQAARslTk0SJwOBFFSXUwk 1313674828
k ALbPjkQAADijSiRISQK9whTL178 1243891806
k AK/VimIAAXI5TwSSMQM8WDKNut0 1325822555
k ALDVimIAASYOTuZNwgVwMgSXZN4 1323792415
k AN7VimIAAM1uT0ZHUA1JGhbBzms 1330020393
b AODVimIAAQzKTuDXaAISoH6BC8M 1323402949

There are more entries, but I don't show all of them.

In the case of Gmail things are somewhat different. With default settings the Gmail POP server only offers newly arrived emails. If there are none, then the popstate.dat file is emptied out, since the server reports that there are no emails available for download from the server.

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If the backend format on the server side remained the same (for instance Maildir -> Maildir) then your IMAP clients as well as your POP clients will probably not care much, if you simply change the hostname in the Account settings.

If however you changed the backend format or did any kind of conversion (for example you converted from Courier IMAP with courier db backend to dovecot with Maildir backend) then in many cases both your IMAP as well as your POP clients will re-download all the messages (or headers if configured so for the IMAP clients). There are conversion scripts (such as this one for cyrus -> dovecot migration) that preserve message ids in order to prevent re-downloading after migration, but even that didn't work out for me and the IMAP clients re-downloaded all the headers.

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