Hey this was the solution... fortunately, I don't have so much users...
Manually, it's effectively a load work ... but it's Ok.
Thx Ovidiu!!

ovidiu wrote:
> Vincent Scribe wrote:
>> Hi,
>> We want to back-up emails... I thought to use filters but filtering
>> emails on dates only works on current map (mails are on IMAP server)
>> ... so not recursively...
>> How can I do that from Thunderbird???
>> Thx in advance for your info,
>>
> maybe filters is not the case. Search messages (advanced search) and
> then do it manually. If the q is not about some automation ..