Depends upon whether you have set up your Thunderbird mail accounts to use IMAP or POP. If POP, then Thunderbird can only access and download from the server Inbox. It is normal to delete the copy off the server after downloading. So, this means if you access that same email address account on another device or via webmail, then the emails you downloaded will not be on server, so you cannot see them. One way to stop this is to make sure the POP mail account leaves a copy on the server after downloading. Tools > Account Settings > Server Settings for pop mail account or right click on pop mailaccount in folder pane and select 'Settings' then select 'Server Settings' for the pop mail account. Select 'Leave messages on server' click on OK Please note that all emails you send will only be stored in Thunderbird, so you cannot view them on the server. Emails deleted in thunderbird will have no effect on emails still on server, unless you have selected that option in the server settings. Alternative, is to create IMAP mail accounts. IMAP mail accounts show a remote view of the folders on the server. you subscribe to see those folders. If you delete an email in a Thunderbird imap mail account, then it is deleted off the server as the folders are in effect, one and the same. So, other devices connecting to server will not see the email that was deleted.