I’m a big fan of Kindle. It is my favorite gadget, I take it everywhere, and recently I found out that Calibre is able to send coverted books directly to my Kindle.
I created a separate gmail account for sending such emails, but unfortunately simple login and password for gmail were not enough.
According to logs it offered me to sign in through web interface and do something within settings.
It was not obvious from the logs, but solution was to go to this page and enable
less secure apps.
Now I can easily send my books to Kindle device!