Who are the “and others” on my list of who the message was sent to? I just sent it to one person! My copy says one person. The recipient’s copy says his name “and others.”

I sent a private message to someone.  He is concerned because on the "To:" section it lists his name and then says "and others."  He tried to click on it to find out who is on that list, but it would not open.  On my sent copy, I do not have the words "and others".  I would like to track down who else received this e-mail message.  How do I do that?  Thank you!