Make BCC default in Thunderbird for multiple or just one recipient
Sometimes you don’t want a recipient of a message to know that you sent the same message to other recipients, or you may prefer always to use the BCC method, even if composing a message to just one recipient, for security reasons. Unfortunately, as you know, Thunderbird won’t provide this option natively, but there this excellent add-on you can use, that lets you change the default option from TO to BCC, and more.
As always, save the add-on anywhere on your PC and from inside Thunderbird Add-ons use the Install Add-on from file option to install it. Then you can configure its options by going to Thunderbird’s Tools > Add-on Options.
The Add-on includes options to just warn you, when you try to send a message without using BCC, change send options on the fly to BCC, make BCC the default option anyway, use a button or menu to change manually all recipients to BCC or otherwise. It can even take action only after a certain number of recipients addressed using the TO option.
The Add-on lets you also avoid the “undisclosed recipients” label on Thunderbird’s “Correspondents” column to let you see the list of your (BCC, TO, CC) recipients.