Susan Tyler has a glitch she can’t sort out:
Since I updated to El Capitan on my Mac, it loads the wrong sender with
the wrong body of the email. I can’t tell you how many times I’ve been
embarrassed to discover when I hit reply without confirming it is the
correct recipient, that a personal email got sent to a business or vice
versa. I do not have this problem on iOS.
Susan was also able to confirm with a third-party email program for OS X that the messages loaded with correctly matched sender and body, so it wasn’t Google that was at fault.
I’ve been unable to replicate this, but checking around, I’ve found reports dating back a few years with similar problems—sometimes in OS X and sometimes in iOS. People were mystified until someone suggested a simple change: Stop syncing contacts with Google.
Read more here:: When Gmail messages in OS X’s Mail show the wrong sender