I'm going to guess that you are sending emails via outlook/hotmail server. There has been no change in Thunderbird. According to google search results Microsoft have been causing no end of trouble for users on different OS and using different email clients. They appear to have created a bug or deliberately have changed how bcc is dealt with, but only if you send emails via their server from any email client with the exception of 'Outlook'. Please read comments, expecially '''6''' , '''16''' and '''24''' in this bug report: * https://bugzilla.mozilla.org/show_bug.cgi?id=1279558 * http://answers.microsoft.com/en-us/outlook_com/forum/opeople-ocontacts/outlookcom-moving-addresses-from-bcc-field-to-the/b4505d03-7b66-4f98-a878-f1d82fcf42a6?auth=1