'''POP mail accounts''' can only access and download emails from the server Inbox. Folders do not synchronise with server. Incoming Emails are downloaded and stored on your computer. You can set up settings to leave messages on the server or remove them. Utlimately, you may need to delete emails off server to reduce quota allowing the server to keep receiving emails. Outgoing emails are stored on computer. All folders are only in that mail account in that profile on that computer. When you delete emails they are only deleted from your mail account unless you have set up settings to also delete them off server. '''IMAP mail accounts''' are different. Imap accounts show a virtual view of what is stored on server. You subscribe to see server folders and copies of headers are downloaded, when you select to view an email, it is downloaded to a cache to facilitate viewing. It is also possible to download full copies which are stored in thunderbird profile folders enabling offline viewing. However, all imap mail account folders synchronise with server, so everything is stored on server, including sent emails. When you delete an email or move it to another imap folder etc, it is also deleted etc on server. The advantage of imap accounts is you can create another imap account on another computer for same email address and hook up to the same server and see all the emails. So, in your case, I would suggest you use IMAP mail accounts instead of POP. Note: Any fully downloaded email copied into the 'Local Folder' mail account will be stored in thunderbird. 'Local Folders' mail account does not synchronise with any server. You can double check to see if emails are fully downloaded into Thunderbird by going into Offline mode. If you can still read them in Offline mode then they are fully downloaded. I would still advise on creating backup copies regardless of whether you use Pop or Imap.