Disappearing emails from server

Problem
If you have configured an email account in Mozilla Thunderbird which can also be accessed using a web-based interface, such as one of GMail or Yahoo! and you are wondering where do your emails go from the main server after you have retrieved them in Thunderbird, then worry no more. By default, Thunderbird is set to delete the emails from the mail server after being sucessfully retrieved.

Solution
To change this setting and make Thunderbird to not delete your precious emails from the server, go to Edit > Account Settings. Under the desired account, choose Server Settings and tick the option "Leave messages on server".