When you send new email, reply to, or forward an email message, one identity is always associated with the email. This is the default identity created as the account name and is used unless you set up different identities. Creating different identities, called personas, allows you to use multiple email addresses from your mailbox. You might want to do this to manage different roles in your job and personal life from this single email account.
You can also add external account addresses to retrieve email from your other POP or IMAP accounts, and you can configure the external account settings so that when you reply to those messages, the corresponding address is in the From field.
Personas, external accounts, and signatures are configured from the Preference>Accounts page and from the Signatures page.
If you set up more than one persona or added external accounts, when you open a new compose window, you can select the identity to use as the From address.