Troubleshooting Why cant't I send some of my epubs?

Hi folks!
I tried sending some of my epub documents to Kindle (through email), but failed. Instead I got emails from Amazon saying that the documents "could not be delivered", but from those emails it is unclear why.
I only have this issue with some of my epub documents. More specifically I find that those generated with the "Wiki Reader" calibre plugin could not be delivered. Has anyone encountered similar issues?
Thank you!