first of all you need 2 email addresses, for example: firstname.lastname@example.org and email@example.com
Allow firstname.lastname@example.org to send documents to email@example.com at “manage your kindle” page.
Go to gmail options and set this rule: IF subject is “firstname.lastname@example.org” FORWARD to “email@example.com”
The sender must write the email content in a text editor and save the file as .txt or .html or other file type accepted by Amazon (for example message.html)
Than the sender should send the email to firstname.lastname@example.org with email@example.com as subject and the message.html as attachment.