I want to move a full year of mail archive offline to "local folders". Neither just copying my IMAP folder to the directory with my local folders works, nor the "ImportExport Tools" addon. What happens is that when I want to open the folder in Thunderbird as local folder, Thunderbird will only show mails up to 24 February and not the rest of the year. The problem is clearly with the msf file: Thunderbirds tries to build a new msf file but fails to complete the job; I can tell because the msf file is a fraction of the size of the original msf file for this mbox folder which resides in the IMAP folders directory. Of course I tried copying this original msf file together with the mbox file to local folders, but for reasons I don't understand, Thunderbird will not use it there. It always tries to build a new msf file which always stops at the same mail from 24 February. (Deleting that particular mail and the one after does not help.) Reading the mbox file with Mbox Viewer works fine, then the full year of mails is shown, so I don't think the mbox file is corrupted. Is it possible that Thunderbird can't deal with this mbox file because it is larger than 2 GB? Why can't it just use the working msf file from my IMAP directory when I copy it to the local folders directory? What can I do? For security reasons I like to take my previous years of mail archive files offline into local folders!