I probably have missed something somewhere that could explain this, and will end up reading your answers and doing

, but here my question:
I have had a lot of trouble with Send-to-Kindle lately, it used to work perfectly fine but now once every two or three times, the book never arrives and I usually don't get any email from Amazon about it.
Yesterday I had some time and decided to try again and see what is going wrong.
I did send an ebook (originally an epub, converted to mobi with Calibre) from my Windows PC's explorer (not my browser) to my kindles with the Send-to-Kindle windows app. I received a while after the email from Amazon saying that my document could not be sent to my Kindle and reminding me of the formats that can be sent to Kindles. It's the first time I have received it in the last weeks, although many books never reached my kindles.
From Calibre, I then tried the "email to" function and it worked beautifully. I immediately received the confirmation of reception email from Amazon. Whispersync and everything works too.
Is this normal, and if so, what have I forgotten ?