Kindle   Before you can send stuff to Kindle you must log in to your Amazon account and find your way to “Manage Your Content and Devices” / “Settings”. On this page find “Personal Document Settings” and within that “Send-to-Kindle E-Mail Settings”. Make a note of the email address for your Kindle - this is the address you must enter in the box above.
Vital   A little lower in the same page you will see “Approved Personal Document E-mail List”. It is vital that you add to this list  PoSP@FastMail.com .
Multiple   This facility only supports one email address at a time. If you wish to send to more than one of your Kindles, try sending to the oldest one. You may well find that the document has become visible to your other Kindles.