1. This guide is for adding aliases to Thunderbird. If you want to use aliases you need to make sure that your public key contains the aliases that you want to use. Click on the button with the three stripes in the upper right corner and then choose Options
2. Choose Account Settings
3. Choose Manage Identities
4. Choose Add
5. Enter the aliases email-address and make sure that the outgoing server is the "main" email-address, then click on OK.
6. How to choose an alias as the sender Choose Write
7. Select "From:"
8. Choose which adress (alias) you want to send from.