How to Export an Email ?

To send eml,

• From Canary, Open the email message > Click 'More' ( bottom right corner at the end of an email ) > Click 'Show Original' > Click Forward.

• From Gmail, Open the email message > Click 'More' ( top right corner at the start of an email ) > Click 'Show Original' >  Click 'Download Original'.  Send the downloaded message as an attachment.

• From Apple mail, Open Apple Mail > Click on the email message download > Click on File > Click 'Save As' from the drop-down menu > Ensure the "Format" is set to Raw Message Source > Click 'Save'. Send the downloaded message as an attachment.

Note : 

1. .txt file can be renamed as .eml. 
2. This is not possible on iOS.