Chance is you have more than one email address. Many Internet users do. Usually one private and one work related email, or a private and public one. Some users, especially webmasters and developers can have lots more than that but if you got more than one you might have experienced a situation where you send someone an email from the wrong email address, say a private email from a work account or vice verse.
PrivateID Reloaded for Thunderbird 3 has been designed to warn the user before the email is send on its way. The extension maintains a list of private email IDs and address books that it will compare all emails with.

It is possible to specify email addresses or strings like domain names to add all of the emails that match the string to the list automatically.
A warning will be displayed if a recipient of a private email is not found in the address books that have been added to the extension’s preferences.

It is still possible to send the email to the recipients but the warning gives the sender the option to rethink that step. It is for instance possible to opt out and change the email account that is used to send out the email.
PrivateID Reloaded can be downloaded from the Mozilla Thunderbird extension repository. The add-on is compatible with Thunderbird 1.5 to 3.0.x.
Update: The script has not been updated for a year and is no longer compatible with Thunderbird. It therefor has been pulled from the official add-on repository.
Related Articles:
Thunderbird Email Address Auto CleanerHow To Password Protect The Email Client Thunderbird
How To Setup Multiple Email Identities In Thunderbird
Thunderbird Email Address Crawler
Create Proxy Email Addresses With Tempalias
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Did you mean PrivateID reloaded 0.1.9.0 with The Firefox add-on link https://addons.mozilla.org/en-US/thunderbird/addon/156144
Yes, thanks and added.
Another useful extension I use for that situation is Border Colors, qhere you can select different border colors for each account.
https://addons.mozilla.org/en-US/thunderbird/addon/4877
Border Colors is nice as well, I have reviewed it in the past ;)
I usually just set my Yahoo mail to use a disposable address as a return address. Than I send my emails with a TO: junk@no.where and all the recipients under the BCC. This way if any of the recipients forward the missive, nobody else’ email get mass distributed along with it.