If this is a POP mail account: Pop mail accounts only access and dwnload from server Inbox. So logon to webmail account via a browser. Create a new folder. Move all emails you do not want to download out of Inbox and into the new folder. This will keep all the old emails on server, but as they are not in the Inbox, Thunderbird will not download them. If this is an IMAP mail account: You can make a setting to choose what messages to download. Right click on mail account name and select 'Settings' or via Menu Bar: Tools > Account Settings or Menu icon > Options > Account Settings click on 'Synchronisation & Storage' for imap mail account select: 'Synchronise the most recent xx days' click on 'OK' see info with images at this link; * https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage This will then only download a restricted amount of emails.