Marskal for Thunderbird sends mail that you choose to the Marskal server that you enter in its options. It has no server of its own and sends nothing to anyone else.

What is sent: when you add a message to your Marskal inbox, with the button in the message header or the message list's context menu, the add-on sends that message's subject, sender, date, Message-ID, and readable text to your Marskal server, which saves them as a task in your account. Messages you do not add are not read or sent.

Why: to make a task of the message in your own task manager, with a link back to the message.

How: only over HTTPS. Signing in uses OAuth with the server; the add-on stores the access and refresh tokens and the server's address in Thunderbird's extension storage in your profile, and keeps no copy of the messages or tasks. Sign out deletes the tokens and asks the server to revoke them.

The add-on collects no analytics, usage, or crash data. What the server does with your tasks is up to whoever runs it, usually you.