You should be able to SAVE the email to a file (.eml) and then attach that file to a new email.
How do you send a Comcast email as an attachment and not just forward it?
To send only one email as an attachment, you would do as jav6joev suggested. To do that click on the elipse in the message header and select the Save option there. But if you want to begin sending all emails as attachments instead of forwarding, click on the gear icon in the upper right corner of the Inbox page, then on Settings>>>>>Mail>>>>>Compose. There you will see the option to send as an attachment.