User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0
Steps to reproduce:
I read the news and saw that it happened again.
Unfortunately it still happens every day; people who send an email to large group of clients but accidentally use the To or CC field instead of BCC.
Although it isn't Thunderbirds fault if users make that mistake, Thinderbird could give users a friendly warning when they press send to help both sender and receiver from experiencing a leak of personal information.
What I propose is to add a warning pop-up when the To and CC field add up to more than 5 addresses. "You are sending this email to more than 5 people who will be able to see each others email address. Are you sure you didn't mean to add these addresses in the BCC field?"
With options "Send" and "Don't send yet"
This warning would be enabled by default, but the threshold can be adjusted in preferences, including the option to disable the warning completely.
I picked more than 5 as an initial default but we could implement this with a different default threshold.