You can set up identities in Thunderbird so when composing email, you can choose an identity the specifies a different From: address than your own email address. To do this:
- Go to Tools > Account Settings.
- With your username@mit selected in the left hand pane, click on the Manage Identities button.
- In the next window click Add....
Result: This produces a tabbed window where you can fill in a name, email address, and reply-to email address.
After you've created the identity, you can choose it from a dropdown list in the Compose window that displays after you click the Write button to start a new message.