Chance is you have more than one email address. Many Internet users do, usually one private and one work related email, or a private email address and a public one.
Some, especially webmasters and developers can have more than two email addresses. If you got more than one you may have experienced a situation where you sent an email to someone or even multiple recipients using the wrong email account.
Say you sent work related emails using your private account, or a work account to sent private emails.
While you can get around this by separating email addresses by program, for instance by using Thunderbird for all private emails, and Gmail or Outlook for all work related emails, it is not that comfortable.
PrivateID Reloaded
PrivateID Reloaded for Thunderbird 3 has been designed to warn users before emails are sent on their way. The extension maintains a list of private email IDs and address books that it will compare against all emails.
Basically, what it does is link email addresses that you use to address books. These addresses are whitelisted then, and you won’t get any warnings if the extension finds a match.
It is possible to specify email addresses or strings during configuration. The latter enables you to match domains so that all addresses are matched automatically without you having to type each individually.
The program displays warnings if recipients are not found in address books linked to the “from” email address.
It is still possible to send emails to recipients, but the warning gives you the option to rethink that step. It is for instance possible to opt out and change the email account the messages are send from.
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 therefore has been pulled from the official add-on repository. There is no comparable extension available at the time of writing that you could use instead to avoid using the wrong email account when sending out emails in Thunderbird.