Do you have any Anti-Virus scanning incoming mail because maybe it's messing up the emails as downloaded or the files were emails are stored. Are you using a Pop or Imap mail account? Have you got any settings which may alter the download eg: Account Settings If Pop - 'Server Setting' are you downloading on headers ? If Imap - 'Synchronisation & Storage' Message Synchronising Is checkbox selected 'Keep messages in all folders for this account.......'? yes is ok Click on 'Advanced' button - are all folders selected to download full copies ? Under 'Disk Space' Have you selected 'Synchronise all messages locally regardless of age' - yes is ok Have you selected the checkbox to 'Dont download if message larger than' ? no is ok It is also possible the file where everything is stored is corrupted. Corruption usually occurs when an Anti-Virus or Malware program tries to scan it or fix something. It can also occur if regular housekeeping eg: compacting of folder is not performed. If you are using an Imap account then everything is stored on the server, so it would a simple case of Removing the imap mail account and any previosuly downloaded data via 'Account Settings'. Select the imap account name in left pane, click on 'Account Actions' and select 'Remove account' Then Exit Thunderbird to ensure all data is properly removed. Wait a few moments, restart Thunderbird and add the imap account again. Check all your Account Settings are as set up as required. New files would be created and emails will get downloaded again.