first of all you need 2 email addresses, for example: email@example.com and firstname.lastname@example.org
Allow email@example.com to send documents to firstname.lastname@example.org at “manage your kindle” page.
Go to gmail options and set this rule: IF subject is “email@example.com” FORWARD to “firstname.lastname@example.org”
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 email@example.com with firstname.lastname@example.org as subject and the message.html as attachment.