How can I hide the Resent-from header in Mail?
Visit https://support.apple.com/kb/PH19118?locale=en_US
The following reads:

When you view messages, you can show more fields in the message header, such as List-ID or Resent-From.
Show the headers: Choose Mail > Preferences, click Viewing, then select “Show additional message headers.”

Change the headers that are shown: Click Select Headers. To delete a field, select it, then click Remove . To add a field, click Add , then enter a field name.