Here’s something that I just ran into this week, although a search of the web shows it apparently has been noticed by some people for some time.
I sent out an email to several people, and used the BCC: to protect the various recipients email addresses. In such a case I normally include myself as an addressee. Much to my surprise, when my copy came back to me, the email header had a BCC: line with everyone in the distribution list shown.
In this case, I was using Thunderbird, and it uses Gmail as my mail server, both inbound and outbound.
I guess I assumed that Gmail, when it passed the email on to the various destinations, would be stripping the BCC information off. But that apparently isn’t the case.
Some testing here showed that when one of those emails was passed from Gmail to Frontier, that the information was still present.