You most likely don't need identities, at least not as Thunderbird uses the term. Most of us would reply to an email address by replying "from" the address it was sent "to". I wouldn't reply to business emails using my private email address, nor would I reply to private email messages using my work email address. Email software generally goes out of its way to maintain this consistency. So you want to override this built-in behaviour. ;-) You might find these add-ons useful: Folder Accounts lets you associate an email address ("identity" in the broader sense) with each folder, so this can force the use of a preferred address. Correct Identity reads through the message you're writing and picks out a relevant "identity" to use as "from:", depending on whom you are writing to. This helps me avoid using my personal email address to reply to business emails and vice versa. It can also flash up a warning if you try to use an "unapproved" email address in the "from:" field. I have used Folder Accounts to override the default identity for one or two accounts which are used in a very specific context, such as reporting spam, but it has clashed with newsgroups. Of course, this may not matter to you. Of the two, Folder Account may suit you best if you don't use Thunderbird to read newsgroups. https://addons.mozilla.org/en-US/thunderbird/addon/correct-identity/?src=api https://addons.mozilla.org/en-US/thunderbird/addon/folder-account/?src=api How to install: http://www.ramsden.org.uk/3_How_to_install_Add-ons_in_Thunderbird.html