Multiple identities per e-mail account
From MozillaZine Knowledge Base
You can tell Thunderbird to use more than one identity per mail account. This would be the case where you have a catch-all account for a domain or forward emails from another email address. The instructions below assume that you have Thunderbird 0.7 (or higher).
- Go to Tools -> Account Settings - Choose the account that you wish to add additional identities to - Click Manage Identities (The button is on lower right) - Add the additional identity that you want. - Repeat as necessary for additional identities
Now, you can select any of the identities when replying to mail or composing a new message.