what type of mail account are you talking about... pop or imap?

If pop: You can set up to delete of server. options: when you download or when you delete an email or after speciafied number of days?

Right click on mail account in Folder Pane and select 'Settings: select; 'Server Settings' To auto delete off server when you download: Uncheck 'Leave messages on server'

If you want to leave on server for a given amount of time in case you need to download again. Select: 'Leave messages on server' select 'For at most eg: 30 days'

or if you want to keep emails but not those you delete: Select: 'Leave messages on server' Select: 'Until I delete them'

Click on OK

If imap: You would need to get a copy into your 'Local Folders' mail account, before deleting off server.

Thanks Toad-Hall. I was referring to imap. I followed your advice and set them to be deleted in 30 days. Thanks Again!

Imap mail accounts show what you have on server. So the imap mail account folders in Thunderbird, are in effect, the same as the ones on the server which you can also view via webmail account using a browser.

If you delete them in Thunderbird, after compacting folder, they will also be deleted off server when synchronisation occurs.

If you want to keep a copy in Thunderbird and delete off server, you will need to get a copy into the 'Local Folders' mail account. Check you can read emails in offline mode (then you know it is properly copied over to Local Folders). I usually synchronise folders for offline use. then in offline mode copy all required emails from imap folders into other various folders in the 'Local Folders' mail account. Then I can delete them from the imap folders, return to online mode, compact folders and remove off server.