Emails do not disappear unless you have a filter that is hiding them or you have deleted them or you have allowed an Anti-Virus product to 'try and fix' something it reports or you have another device removing them off the server etc. The latter being the most common culprit. Please note - it is quite normal to discover emails being deleted from 'Spam' and 'Trash' folders if you are using an imap account as most servers will empty those folders automatically every x days. How many different ways do you read your emails? Do you view emails only via Thunderbird on one computer? OR do you view emails via a phone as well ? OR do you sometimes view emails via logging on the webmail account via a browser? In Thunderbird is the account set up as a POP account or an IMAP account? A couple of Example Senarios: If you have an imap account in Thunderbird and an imap account in another device eg: phone and you delete something off server via the phone, then that email does not exist on server, so Thunderbird cannot display it. If you have an imap account in Thunderbird and a pop account in another device eg: phone and phone is not set up to 'leaves messages on server' then the phone has popped the emails off the server upon download. Therefore the emails do not exist on server, so Thunderbird imap account cannot display them. So it's important to let us know about all the various ways you are accessing you emails and the type of mail account.