I was using Calibre's email feature to send "News" to my Kindle. As usual it was working fine. Just make sure the setting you are using for smtp are good and server can accept them. Try to test it - go to "Preferences" -> "Sharing books by email" and press "Add email" in order to add some other real email you have access to (which you can check so you don't need to sent test email to amazon's
user@kindle.com). Then select this new email in the list and press "Test email" button. If all good and you received your test email - Calibre is configured fine, if Calibre rises an error - it's something you need to resolve (depends on the error).