Sylpheed stores its data by default under ~/Mail and indeed this folder exists and even contains empty subfolders such as inbox, sent, draft etc.
However I cannot find any mail storage files even after downloading 800 messages form my IMAP mailbox. The messages are available offline so it seems they were downloaded successfully.
Any hint as to where I can find the local storage?
Perhaps the problem is IMAP itself. There is POP and there is IMAP. POP is used to download email, and the downloaded messages can be deleted from the server. IMAP generally is not used to download anything. It only syncs your headers and does not use your disk space. If you want to download your email, store it and flush the email account online, use POP. Then you will find the messages somewhere. Beware that some software save every email message as a separate .eml file but others have big one-piece database files that may be only a burden to their owners, and get unusable without the software that has generated them.
