In Outlook, the Forward as Attachment feature provides a way to forward complete copy of one or messages. When this feature is used the forwarded message appears as an attachment to the recipient and preserves important details that would be lost when forwarded normally.

Select the message(s) you want to forward.
Click the More actions menu (...) in the toolbar.
Choose Forward as Attachment.
(If you don’t see it, select multiple emails — the option appears in the toolbar.)
Gmail doesn’t have a one-click “Forward as attachment” for single emails, but you can:
For multiple emails:
Go to your inbox, check the boxes for the messages.
Click the three-dot menu and choose Forward as attachment.
For a single email:
Download it (More → Download message) and then attach that .eml file to a new email.
Select the email you want to forward.
Go to Message → Forward as Attachment in the menu bar.
A new message opens with the email attached.