Additional identities are helpful when you want to send mails in the name of a mailinglist or when you want to use an alias aditional to your default mail address

1. Hit the alt-key and go to Tools > Account settings and select your account, where you want to add a new identity. Click on Manage Identities.

2. Click "Add" and fill out the required data. This means in particular your name, as it is meant to be shown to the recipients of your mails, and your mail alias or the address of the mailinglist in whose name you want to send your mail.

3. When you have added your new identity you are able to select your new identity by selecting it in the sender field when writing a new mail.