If you use some formats of email -- such as my Hotmail account -- you can set it so it shows you every stop along the way an email took to get to you. Servers and so on. It can be made to print out above the body of the email.
Also, even if you use the "blind copy" feature (which is the polite thing) when you send out a mass email, there are various text editors & I assume other programs that, when you open an email or transfer an email in them, will show every address it has been sent to.
So even "bc" is not really secret. If you don't want a person to know the others you've sent your email to -- such as when you send out a resume or job application to several employers via email -- you'd best send each individually.
I am sure someone with some actual technical understanding will tell you more. Have you got a spam or harassment problem, or do you need to assure confidentiality of your own outgoing email?