Email identities can be created in the account settings of the Thunderbird email client. The account settings can be reached from the top Tools menu. The user then needs to select one of the configured email accounts. A button Manage Identities should appear in the menu that is also displaying various information about the selected email account.

In there it is possible to add or edit email identities. It is for example possible to select a different SMTP server, signature or vcard, change mail folders, use a digital signature and specify a different email address. The email address can be the same but it does not have to be. It is even possible to add a completely unrelated email address for that new identity. You could theoretically configure the identity to send email that looks like its coming from any email account in the world (enter test@example.com for instance and send the mail to one of your accounts to see how it works).

When you write a new message, Thunderbird uses the default identity for the account you are working in. When you reply to a message, Thunderbird uses the identity that matches the address the message was originally sent to, if it can find a match in the account you are working in.

You can select any identity from any of your accounts when replying, forwarding or composing a new message by selecting it in the From drop down list box. If desired, you can also create a unique signature, specify the draft folder, what folder your copy of the sent message should be stored in, and what reply-to address should be used for each identity.

Additional information about email identities in Thunderbird are available a the Mozillazine website.