How to migration my mails from another server ?

[Mik J]

Hello,

I would like to host my mails and I would like to retrieve my existing mailbox content from my ISP

My ISP
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I don't have root access on the server
The server seem to use Dovecot according to the banner.

My machine
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I have root access on the server
I use Dovecot 2.2.21
I use Maildir



I read this page multiple times but it's not clear to me
https://wiki2.dovecot.org/Migration/Dsync

"Set up configuration for the IMAP server you wish to migrate from"
Is it my ISP's server or my server ?
In which configuration file should I do this ?

The same for this option 
dsync_features = empty-header-workaroundwhere is it supposed to be configured ?


My most important case is quite "simple": the mailboxes (there are 2 or 3) I want to migrate don't exist on my machine, so there is no such problems as merge existing mails.
Will this migration use imap and retrieve the content of the mails ? Will it use port 143 or 993 ?
I'll use the backup option


I have another case which there are mails on both servers. So I would like to keep existing mails on my machine.
I'll use the sync -1 option ?

Thank you