I see it this way: in the account settings, the user specifies the preferred format for exporting letters (msg, txt, html). In the list of letters (including in the search results), the user has the ability to select individual letters or entire chains, and click "Export". After that, compression with minimal compression will be performed on the server side and the user will be asked to specify a location on the computer to save the zip archive with letters and attachments, each letter in a separate folder with the name [yyyy-mm-dd hh:mm:ss %subj% by %sender%].