I'm assuming you are using an IMAP mail account to see emails. When you Archive an email, it is usually moved to an 'Archives' folder within the same imap account. So it is still on server, just in a different folder. In the case of gmail, they auto use the 'All Mail' folder as the archive. How to check where are they archived? * Right click on imap account name and select 'Settings' * select 'Copies & Folders' * Under 'Message Archives' section, it will say where Archived emails are stored and usually this is 'Archives folder on mail account name'. If you cannot see the 'Archives' folder in the Folder Pane, perhaps it is because you have not subscribed to see that folder. * Right click on imap mail account name folder in Folder Pane and select 'Subscribe' * Select 'Archives' folder and click on 'Subscribe', click on OK. If you need to get emails off the server to gain space, then putting them in Thunderbird's 'Local Folders' mail account would be a good idea. As you would still have access to them. However, you do need to make sure you have full copies downloaded into the imap account. So check settings: * Right click on imap account name and select 'Settings' * Select 'Synchronisation & Storage' Message Synchronising: * Select 'keep messages in all folders for this account on this computer' * Click on 'Advanced' button and make sure all the folders and subfolders are selected, so full copies of all emails in all folders get downloaded. Then Click on OK. Under 'Disc Space' * Select 'synchronise all messages locally regardless of age' * Select 'Don't delete any messages' * click on OK Now you need to force a full synchronisation of all folders and go into offline mode. So do this: * Menu app icon > File > Offline > Download/Sync now A small window opens * Select 'Mail Messages' * Select 'Work offline once download and/or synchronisation is complete' You can easily check which folders will be effected by clicking on 'Select', but you should find all are selected. * Click on OK and download sync will commence. When it has complete, Thunderbird will go into offline mode and you will see the 'two blue screen icon' in bottom left Status bar change to a 'two black screen icon'. Now you are offline. It is best doing a large move/copy of emails in Offline mode to prevent any synchronisation of folders whilst you are doing a load of work with them. If you can select emails and still read them in offline mode then you know you have successfully downloaded full copies. * In 'Local Folders' account, create suitable folders ready to receive emails. * Highlight/select batches of emails in the imap account, right click on highlighted emails and use 'Copy to' and select the appropriate folder in 'Local Folders'. I recommend using 'Copy to' because if it fails for any reason, you will still have originals, so can try again. Once all the emails you want to move have copies in 'Local Folders' mail account, you can delete them from the imap account folders. Remember, if you can read emails in Local Folders account then they are all copied ok. So check before finally deleting from imap. Finally, compact the folders in the imap account to fully remove hidden 'marked as deleted' emails. * Right click on eg: Inbox and select 'Compact'. Once all imap folders have been compacted, the folder size will shrink. Now you are ready to go back into online mode. But at this point, I would advise you create a backup of your profile because you have only the one copy. In Thunderbird. * Help > Troubleshooting Information Under 'Application Basics' section * click on 'Open Folder' A new window opens showing the contents of your profile name folder. * Exit Thunderbird now - this is important. You need to go up the directory. Currently you are in the 'profile name' folder - usually called xxxxxxxx.default' where the x's are letters and numbers. This is the default location where profile name folder resides: * C:\Users\<Windows user name>\AppData\Roaming\'''Thunderbird'''\Profiles\<Profile name> * You need to go up to the 'Roaming' folder and copy the 'Thunderbird' folder and store it in a suitable location eg: external hardrive. Then start Thunderbird. If you do not want to create a backup right now - which I do not advise - then clicking on the 'two black screen icon' in Status bar will return it to a 'two blue screen icon' and you are back in online mode. Folders will synchronise with server. After a while, logon to the webmail account via a browser to check the emails have been deleted off server.