stateless synchronization between two imap accounts

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stateless synchronization between two imap accounts

I used to be an impasync user. But one day this piece of software had 
been removed from debian repos by its author. I'm now trying to find an 
alternative to imapsync. I currently paying a strong attention to 
offlineimap and mbsync.

I try to reproduce the same behaviour as in imapsync : stateless 
synchronisation between two imap accounts.

One of these imap accounts is the master account, the other one is the 
slave account. The master account is the one that I use on a daily 
basis, as my main mail account. The slave account is juste a backup 
account synchronized from time to time from the master account. This 
solution make it possible to have a secondary backup mail account in 
case of hardware or software failure on the master account. It is even 
useful in case of accidental deletion of mails on the master account, 
which might be backuped on the slave account (if created before the last 
synchronization).

The stateless statement is important for me, as I want to be able to 
synchronize my two imap accounts from various computers, even if I 
reinstall the system, leading to restart from a fresh install without 
cache data.

If I consider a piece of software such as imapsync (for imap 
synchronization) or weex (for ftp synchronization), these softwares 
might use caching or not, they always have a "rebuild" mode that lead 
the software to build or rebuild its cache data. This is permitted by 
the fact that one account is the master and the other one is the slave. 
As there is no two-way synchronization, it is easy to rebuild the cache 
data.

How does offlineimap determine if a mail has been deleted from an 
account, or newly received from the other account ?

It appears that offlineimap seems to be a stateful piece of software. 
As offlineimap maintains a cache of all mails it knows, offlineimap is 
in a way a stateful application, while imapsync is stateless. As 
imapsync does only a 1-way synchronization, it is easy for imapsync to 
be stateless : imapsync only has to compare mails from the two 
repositories and synchronize them considering that the original 
repository as the reference repository.

Considering offlineimap, what happens if the cached data of offlineimap 
are deleted ? Can the synchronization be recovered ? In this case, how 
can offlineimap determine if a mail has been deleted from an account, or 
newly received on the other account ? Cached timestamps are helpless 
whitout cached data or cached metadata ?

In my case, I use the new parameter "readonly = True" ? Is there a 
special way of rebuilding cached data ? As one of the repository is 
readonly, every mail deleted from this repository should also be deleted 
from the other repository. Does OfflineIMAP work like that ?