Right-click the email you wish to send and select Forward As → Attachment → Send.
Select the message you wish to forward → Home → Respond → More → Forward as Attachment → Send.
Select the Mail applications on your Mac. Choose the message that you wish to send as an attachment → Select Message from the top menu → Forward as Attachment → Send.
Select New message → Drag the emails to the text box to be sent as an attachment → Select Send to send the message with the attached e-mail to the recipient.