But after Synchronization & Storage was set ...

What exactly was set in Synchronization & Storage?

... the client ... downloaded all messages from the remote server.

This is how IMAP works.

Even though all the historic messages were deleted ...

Assuming you understand how IMAP works, then you're aware that this would also delete those messages on the server for good.

... the client downloaded all the messages again next time.

How do you determine this?

just download recent messages for 30 days was set in Synchronization & Storage.

It doesnt make any sense for your other questions and answers.

With IMAP, what exists in the server's IMAP folders is what you will see in Thunderbird. The actions you take on messages shown in Thunderbird gets mirrored or reflected on the server as well. For example, if you delete a message in Tbird, the message gets deleted on the server too. IMAP keeps Tbird (client) and IMAP server in sync. With IMAP, you'll always see ALL the messages that are on the server, regardless of their age. This is achieved by downloading the headers of ALL the messages that exist on the server. The setting you applied does not affect what you see in Thunderbird when using IMAP. It only affects which messages get downloaded in full, so that they get stored on your computer's storage for offline access.

Note that emails are made up of two major parts; the header and the body. In case of IMAP, headers are downloaded for ALL messages that are on the server. The body of those messages, however, are only downloaded as per the Synchronization & Storage settings you specified. By default, Tbird is configured to download full messages (header+body) for all IMAP folders so that you can access them while offline. You can, however, choose individual folders whose messages you want to be downloaded in full. You can even entirely disable donwloading of full messages for ALL folders, so that only headers are fetched. In that case, the rest of the message (body) gets downloaded only when you open that message, and Tbird uses the cache to temporarily store the contents of the downloaded message.

Thanks Stans, your answer is very clear. But a further question about 'For example, if you delete a message in Tbird, the message gets deleted on the server too. IMAP keeps Tbird (client) and IMAP server in sync. '.

Even though the options 'keep message on server' or 'never delete messages' (I cant remember the exat option ) is set, are the upper actions still be made by Tbird? Many thanks.

Alex

options 'keep message on server'

With IMAP there is no such option. Messages are always managed on the server.
This option only exists when using POP.

'never delete messages'

This needs to be put into context. With IMAP it means there is no automatic clean up of older messages. Never.

With IMAP, what exists in the server's IMAP folders is what you will see in Thunderbird. The actions you take on messages shown in Thunderbird gets mirrored or reflected on the server as well. For example, if you delete a message in Tbird, the message gets deleted on the server too. IMAP keeps Tbird (client) and IMAP server in sync. With IMAP, you'll always see ALL the messages that are on the server, regardless of their age. This is achieved by downloading the headers of ALL the messages that exist on the server. The setting you applied does not affect what you see in Thunderbird when using IMAP. It only affects which messages get downloaded in full, so that they get stored on your computer's storage for offline access. Note that emails are made up of two major parts; the header and the body. In case of IMAP, headers are downloaded for ALL messages that are on the server. The body of those messages, however, are only downloaded as per the Synchronization & Storage settings you specified. By default, Tbird is configured to download full messages (header+body) for all IMAP folders so that you can access them while offline. You can, however, choose individual folders whose messages you want to be downloaded in full. You can even entirely disable donwloading of full messages for ALL folders, so that only headers are fetched. In that case, the rest of the message (body) gets downloaded only when you open that message, and Tbird uses the cache to temporarily store the contents of the downloaded message.

My question is a little different from the original topic, but covered exactly in this response. I have TBird set to IMAP to Synchronize ALL messages regardless of age. However, if I respond to an e-mail on another computer using Outlook, then TBird does not capture or synchronize the "SENT" messages from the Outlook server. I can go back to Outlook and they are all there. But in TBird, those replies are not showing. Any reason it wouldn't synchronize ALL of the messages from the server?