I'm sure I'm not alone in occasionally forgetting to specify BCC when send emails to a contact group - until you've just hit Send, of course! For example, if a contact group is made up of people whose privacy is very important to them (eg people with mental health issues, ex-convicts etc), the mere fact of sending a CC group email, however innocuous in itself, is a considerable invasion of privacy. Even for uncontroversial emails, many people understandably resent having their email addresses made public, even to a limited group, unless they know and trust the rest of the group. I feel that email clients should make BCC the default for group emails. If Thunderbird can include a warning of missing attachments - a relatively trivial annoyance - this seems to me of far greater significance.