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               Imapsync tips about POP3.
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Questions answered here are:

Q. Can I use imapsync to migrate emails from pop3 server to imap server?

Q. How can I handle those terrible POP-leaving-a-copy users?
   POP-leaving-a-copy clients will download a new copy of all 
   their emails after a migration, which is a big pain and 
   causes hundreds of support calls. 

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Q. Can I use imapsync to migrate emails from pop3 server to imap server?

R1. No, but you can migrate emails from a pop3 server to an imap server 
with the command line tool pop2imap:
http://linux-france.tk/prj/pop2imap/
http://linux-france.tk/prj/pop2imap/README

R2. Yes, sometimes, because many pop3 servers runs in parallel
with an imap server on exactly the same mailboxes. They serve 
the same INBOX, ie, imap serves INBOX and several other folders, 
pop3 serves only INBOX.
So have a try with imapsync on the same host1.

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Q. How can I handle those terrible POP-leaving-a-copy users?
   POP-leaving-a-copy clients will download a new copy of all 
   their emails after a migration, which is a big pain and 
   causes hundreds of support calls. 

R1. Context and condition: you have access to the POP email client tool
and the user WILL STAY using POP.

POP email client tools can leave or delete the messages after pulling 
them. It's very often a configurable behavior. Configure the email client
tool to delete the messages already pulled and then they won't never 
be pulled again from the server to the client. 

If you can't access to the POP email client tool, see R2 below.

Problem in the future: INBOX will be seen as empty when the email client
go to IMAP (unless the email client tool allows then an upload
synchronization, but don't count on that...)


R2. Context and condition: you know which users use POP.

If the pop mailboxes are known then all the INBOX messages can be moved 
in an subfolder called INBOX_backup for example. Ti help you do this,
the tool pop2imap transfers messages from POP to IMAP, without duplicates, 
without deleting the messages transferred, by default, and can put them 
in any subfolder you want to. It can also delete them and, that way,
do what the email client tool don't or can't do, for any reason. 
We're here to do a good job with what we have, do we?

In case of Dovecot for example, the magic can be there since already 
fetched messages via POP are seen as \Seen in IMAP but the new messages are 
seen as new messages, thanks to the Dovecot developper Timo's cleverness!. 
So it is possible to sync back only the new messages to INBOX, 
with "imapsync ... --search UNSEEN ...", and then  POP users won't see the old 
messages come back but they will see the new messages and fetch them via POP.


R3. Context and condition: you know nothing

Do R2 without the sync back of UNSEEN messages. It can't hurt to have a backup
of INBOX in a subfolder for IMAP or POP users.

R4. Here are some points, an analyse that made me purpose the not 
to bad but not very good R2 answer.

There was a discussion about this issue on the imapsync mailing-list:
http://linux-france.tk/prj/imapsync_list/msg02622.html
http://linux-france.tk/prj/imapsync_list/msg02623.html
My reply was not very good
http://linux-france.tk/prj/imapsync_list/msg02624.html
It's because I wrote pop2imap and it uses a stateless
mechanism to avoid duplicates, I wasn't aware of UIDL when I wrote pop2imap,
and later when I entered the discussion.

See also Rick Sanders discussion about this issue:
https://www.emailquestions.com/threads/how-to-prevent-duplicate-emails-after-server-migration-for-pop3-users-with-leave-mail-on-server.8109/

Quoting Timo Sirainen on
https://wiki2.dovecot.org/Migration

"If a client already saw changed UIDLs and decided to start
re-downloading mails, it's unlikely there is anything you can do to
stop it. Even going back to your old server is unlikely to help at
that point."

"Some (many?) POP3 clients also require that the message ordering is
preserved."

"Some clients re-download all mails if you change the hostname in the
client configuration. Be aware of this when testing."

Quoting 
https://wiki.dovecot.org/POP3Server

"Flag changes
By default when a message is RETRed, \Seen flag is added to it. 
POP3 itself doesn't support flags, but if the mailbox is opened 
with IMAP (eg. from webmail) it's shown as seen."

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F1l3 Manager
