I'm trying to understand exactly what you did. Most people with Imap mail accounts will have an imap subscribed 'Archive' folder which is within the imap mail account. So in effect, all emails are still stored on server, but they have been moved into a different folder called 'Archives' which may have subfolders perhaps by year. It is also possible to archive emails to a folder outside of the imap account and storing them in your 'Local folders' mail account which is now storing them on your computer. This has the effect of deleting emails off the server. You would need to set this up in the 'Account Settings' for the imap account. * Right click on imap mail account in Folder Pane and select 'Settings' * select 'Copies & folders' Message Archives section * click on 'Archive Options' button * Select relevant options eg: Yearly archived folders If you want to keep your current folder structure, then select the option 'keep exisitng folder structure of archived messages' This means if you archive emails from eg: 2018 and some are in the 'Inbox' and some are in 'Sent', then Archives folder will look like this: Local Folders > Archives > >2018 > > > Inbox > > > Sent If you selected 'A single folder' and 'keep existing folder structure' Then you may see something like this: Local Folders > Archives > > Name of folder * click on OK. * Select 'keep message archives in:' * Select 'Other' and choose 'Archives on Local Folders' * Click on OK From then onwards when you select an email and click on 'Archive', the email is moved to the designated folder. Please note that in a gmail account by default Archive means remove all labels and keep email in the 'All Mail' folder. The 'Archive Options' button in Thunderbird will be deactivated. It is a good idea to make sure you have full copies of emails already downloaded / full synchronised and up to date in folder before archiving to a location outside of the imap account. When ever you move emails from one imap folder to another imap folder or out of the imap account to Local Folders mail account, there is a load of communication going on with server to accomplish this. This is because the imap folders are synchronised with server folders - they are in effect one and the same. If you have an Anti-Virus product that scans files when opened or scans internet uploading/downloading of files then this can cause a slight slowdown in communication. You cannot move a 'folder', but you can move emails off server to 'Local Folders' mail account and back again. Closing Thunderbird whilst communication is occuring is not advised as emails may be in transit. '''Re:''' ''canot delete a folder in an imap mail account:'' You need to unsubscribe from seeing server folders. You cannot delete any default folder. Other folders can be deleted, but in some cases you need to unsubscribe from seeing them and then delete them via webmail account. Your explanation sounds like you move a 'Local Folders' folder to somewhere it could not go and whilst Thunderbird was trying to process this, you closed the application. You could check to see if there is anything in the 'Local Folders' mail account. In thunderbird Help > Troubleshooting Information click on 'Open Folder' button a new window opens showing the contents of your profile name account - when you create backup, this is the folder you would have been backing up to an external location. The 'ImapMail' folder will contain your imap mail accounts. But you want to check 'Local folders' so.... click on 'Mail' folder click on 'Local Folders' mail account name folder There should be an 'Archives.sbd' folder and it contains the 'Archive' subfolders. mbox files have no extension and they contain emails. Any .msf file is just an indexing file and does not contain emails. mbox files can be opened using a text editor program like Notepad++. Emails are written to the file in the order downloaded, so oldest is at the top. If you make sure you have the window set as 'View > details', the you can enable the 'Size' column and this will help to show you what files have a good size.