[catsittingstill]

{later edit} Oh, hmm. It looks like Francesco had better suggestions as I was typing this long post. I'll leave the post in for posterity. Good luck with fixing the problem. {/later edit}
I don't actually know much about this, but looking carefully at the output that "STMPServerDisconnected" part makes it look like either your computer is not sending out the e-mail (or quit in the middle) or Amazon's e-mail server did not receive it properly.

Here's what I would do:

If you've only gotten this message once, I would just try again; sometimes things glitch.

If you've gotten this message more than once, I would try using calibre to e-mail yourself a different book. That lets you figure out whether it's just that book (something wrong with the book file, maybe?) or whether the problem is more general.

If calibre can't e-mail your Kindle any books, I would try e-mailing something else to your Kindle, without using calibre (use your e-mail program to e-mail a word doc to your Kindle, for example, to be converted by Amazon and loaded via whispernet.)

That lets you figure out if the problem is calibre, or your computer e-mailing to your Kindle in general.

If your computer can't e-mail the Kindle generally, use your e-mail program to send a e-mail to a friend who will let you know if it got through.

That lets you know if your computer can't send e-mail at all, or if it just can't send to the Kindle.

If your computer can't e-mail your kindle (or can't send e-mail at all), then I would try a different site (The Baen Free Library has free books in Kindle format; you could go to the Amazon web site for managing your kindle, greenlight Baen as a sender, go to the Baen Website, e-mail your kindle a free book.

That will tell you whether the problem in e-mailing your Kindle lies in your computer or Amazon's. This is arguably kind of a redundant test if your computer can't send e-mail at all, but while you're working to fix your e-mail it would be reassuring to know that once your e-mail is fixed the problem with e-mailing your Kindle will be fixed too.

Once you know more about where exactly the problem lies, call Amazon if the Kindle can't receive e-mail from anywhere, or if your computer can send e-mail to your friends, but not to your Kindle. Amazon customer support is pretty good.

If your computer can't send e-mail at all, I think you'll need tech support for your computer.

If it's just calibre that can't e-mail your kindle, but your computer can, you need help with calibre settings that I can't give, but I'm sure there's someone here who can.

If it's just the one file that calibre can't e-mail, I would delete the file and regenerate it, though maybe there are fixes that are less work.

In any case, the more you can tell people about where the problem lies, the easier it will be for people to help you.

Good luck!