If you have created an '''imap''' mail account in Thunderbird for the email address then it will show you what is on server because the folders synchronise with server. It is like a virtual view of server. If you have also got an imap account on phone or link directly to webmail and choose to delete an email, then that email is deleted off server. When you start Thunderbird,folders synchronise with server and it cannot show what does not exist on server. If you have a pop mail account in Thunderbird then it can only download what is in the server Inbox. So if you used an imap or webmail link via phone or other computer to server and deleted email off server then it is no longer in the server Inbox and therefore cannot be downloaded to pop mail account in Thunderbird. If you create Pop account in Thunderbird then you would need to alter settings to 'Leave message on server' so that it is not removed after Thunderbird accesses server. Then the email will be available either via webmail or another pop or imap access. In Thunderbird: * Right click on POP mail account name and select 'Settings' * Select 'Server settings' * Select: 'Leave messages on server' * There are a couple of additional options if you want to delete after a certain period of time. * click on OK If you do not want the phone to delete emails off server then you need to use POP and choose settings to leave on server. Then when you delete an email off phone, it will not delete off server. If you are choosing to use Pop and never delete emails off server, then emails will accumulate on server and you may eventually reach your full quota. If quota is full then server cannot receive emails until some are deleted to make space.