If you have an Anti-Virus product scanning then this could be causing the issue. But, I'm just wondering about how well you organise and maintain your Thunderbird. For example... Do you keep everything in Inbox and Sent folders? Do you compact folders on a regular basis to remove all traces of marked as deleted emails? It is useful to have the following enabled so you can see the size of your folders. * 'Menu icon' > 'Options' > 'Layout' > select 'Folder Pane Columns' * Then enable the 'Size' column. see image below as guide. Try this: For any emails more than one year old, set up Archiving options and archive those emails. Including Inbox and Sent folders. Archiving Options: * https://support.mozilla.org/en-US/kb/archived-messages For other emails less than a year: * Create several folders with suitable names for organising emails. * Move emails from Inbox into those folders. * Delete anything you do not want to keep. When Inbox is empty: * right click on Inbox folder and select 'Compact Folder' '''Info on compacting - what it does and why it is important:''' * http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders '''Junk folder:''' * right click on Junk folder and select 'Empty Junk' Notice how the size has not changed. * right click on Junk folder and select 'Compact Folder' Notice size should now say 0