My imap server has long responses for particular messages or there is other problem. Simply I need to download messages to local folders so that i can work with them (as it was in Outlook). Because if i work directly on online account - every move of a small message takes about 10 minutes and that is not possible to do it this way. Or I can try other server, but I am not sure if problem will not remain. I would urgently need really offline operation with messages.
if you haven’t done so already, you can enable Local folders in Tools > Settings > General and check the “Show local folders” option.
Then you can simply Drag&drop all your folders and messages inside them.
Or if you want to still sync with the server but have your messages saved locally, you can download your messages for offline use in Tools > Accounts and under the IMAP tab of your imap account check the “Download messages for offline use” and “Include attachments and images” options.