Mail handlers can malfunction. This Outlook problem is a good reminder of some general cautions with blind carbon copies. If the disclosure of the bcc's to the other recipients will cause sever problems, don't use the bcc. Instead send a separate new message to each intended bcc. By the way, edit out any headers that might be included in the copied text.
Some people claim that blind carbon copies are somewhat unethical.
The bcc's can be used in an unethical manner but there are many ethical uses for them. I use bcc's to help ensure the privacy of my FYI mailings' recipients. Many people use bcc's on some email systems to ensure they get a copy of the email they send out. In many organizations, the bcc can be used to keep key people posted on a matter without having a big list of people displayed upon the email. There are some distinctly questionable uses of bcc's.