I'm not familiar with a way to sync ''directly'' between two installations of Thunderbird. (I'm not a Thunderbird user myself.) If you set up your email account in Thunderbird using an IMAP connection, then your mail folders live on the server with only a local "mirror" copy. IMAP folders can be accessed from multiple devices and changes on one device will sync with the server and then with the other devices. At least, that's the theory. For more on IMAP, see: [https://support.mozillamessaging.com/en-US/kb/imap-synchronization IMAP Synchronization | How to | Thunderbird]. On the other hand, if you set up your email account in Thunderbird using a POP connection, then you have a copy of the messages independent of what is or was on the server, and you have a local Sent Items folder that isn't stored on the server. A device using a POP mail connection is inherently out-of-sync with the server and with any other device connecting to the same mailbox. If you need to switch from POP to IMAP, that's a bit tricky, but might be a solution if your mail service supports IMAP. (It's not something I could describe in a couple of paragraphs.)