When you start the program for the first time, it will ask you to configurean email account.
1. Fill in the user information accoring to the image.
2. Program will try to load configuration automatically but will fail. When it has tried to configure the account enough many times it is possible to choose to configure account manually. Choose Manual Configuration.
3. Choose Server Settings. Fill in mail.shellit.org as Incoming Server and 993 as Port. Choose SSL/TLS enryption.
4. Choose Outgoing Server. Fill in mail.shellit.org as server and 587 as port. Choose checkbox Use User Name and Password and fill in your email address. Choose STARTTLS as encryption.
Now you can use your email in Thunderbird.