@d56alpine You can't send KFX files via e-mail
Just like you can't send AZW3.
And the reason why Amazon sends those verify request e-mails is that you're probably using kindle email address that Amazon considers "unsafe" because it was derived directly from you Amazon account email address:
like
name_surname@whatever.com >>>
name_surname@kindle.com
Go to amazon.com/myk, login in asked to do that and under Devices edit the email address associated with that Kindle device you want to send books to and add something extra like:
name_surname
_mykindle1234@kindle.com
Then in the future use that new Kindle email address (name_surname_mykindle1234@kindle.com) and you shouldn't need to verify.