Basically this question - https://www.reddit.com/r/Calibre/comments/tcew2i/howcan_calibre_send_books_to_my_kindle_using_my/
I recently set up my Kindle to receive books by email. The only verified address is my personal email. In Calibre, I set up the send-by-email feature by providing my Kindle's "email address" ([email protected]) and I filled in the "from email ID" field (my personal email). I did not provide any optional server or my email password. I am not logged in to my email on any email client on my Desktop.
Calibre is able to send books to my Kindle. How is this possible? I also do not see any sent email from Calibre in my personal email account.
I do feel secure that I do not see any sent emails and that Amazon asks me to verify any book that gets sent to my Kindle. But I just want to know what protocol is making this possible? Is there something happening in the backend that's enabling this?
In short, yes. Emails can be sent by any programming language, including python used by Calibre. As long as it has a target email server to send it to (whom then handles delivering the actual message). By filling out the following items in the Calibre Preferences-->"Sharing Books by Email" Settings, you're telling it exactly where and how to send the SMTP message with the book attachment:
Hard to say without knowing which system (Hotmail or Gmail?) you're using, but best guess if you truly didn't fill these in, then it may be grabbing MSN/Outlook/Hotmail settings from your Windows credentials, although I would still expect you to have to sign in at some point. In Windows 10, programs can query the current logged-in user, and their (public key equivalent) credentials. Other apps like the outlook mail server can then "trust" things sent by these credentials (as long as they've been logged into within a period of time), without further authentication.
I'd say check these Calibre settings and see which server(s) it's pointed to. If it is MSN/outlook, then you can also check settings on the "Credential Manager" in Windows. Look for one starting with "MicrosoftAccount:xxxxxx". These are built to "automatically respond" in a secure way to asks for credentials by applications.
In Depth on Windows Credentials:
https://learn.microsoft.com/en-us/windows-server/security/windows-authentication/credentials-processes-in-windows-authentication