Quoting Robert Schetterer <rs@sys4.de>:

Am 04.09.2018 um 17:18 schrieb Sami Ketola:

On 4 Sep 2018, at 18.00, Robert Schetterer <rs@sys4.de> wrote:

Am 04.09.2018 um 16:52 schrieb Sami Ketola:

On 4 Sep 2018, at 17.47, Robert Schetterer <rs@sys4.de> wrote:

Am 04.09.2018 um 09:41 schrieb Sami Ketola:

imapsync always loses data

never saw this, be carefull by anounce such myths

It is a fact. Imapsync works over IMAP only and IMAP protocol does not even support transferring all data. At most at least UID numbering will be lost and end users need to invalidate their local caches.

but using "looses data" might others think it also may fail with imap,
so be more detailed next time

UID number is also data that is saved on the IMAP backend. If that is lost then it's "lost data".

Sami

Sorry i migrated terrabytes of mail with imapsync and never had a
problem, it works as designed, also with maildir rsync did a good job,
what never worked as it should was dsync ,cause of bugs ,that may
changed now

so this is my answer to topic

"Best way to move mail from one server to another"
 

https://imapsync.lamiral.info/FAQ.d/FAQ.Duplicates.txt

Seems to use UIDs so that 'data' isn't lost.