Kindle conversion email vs Calibre

Hi everyone!

I'm really so annoyed by this, and I hope someone can help me. Normally, what I used to do was that I'd convert my epub books to mobi, then from my own email address send the file to my Kindle email address and then it would appear on my Kindle. I have the latest Paperwhite. This was convenient, as sometimes I read on my phone while out an about, and my devices would all sync to the latest position. However, for a while now, this hasn't been working. Whenever I try to do this, I get the general infuriating and unhelpful mail back of conversion having gone wrong. I have no idea why. If I can get the book in mobi, and skip Calibre, it works fine. The moment I put it through Calibre, it fails. I have mocked about with the settings for ebooks on Calibre, maybe that's what's inhibiting me from doing this? I've combed through dozens of forums, and there is no solution anywhere. I could use a USB, but for one, Calibre doesn't put it in the right directory on my Kindle anyway, so I can't use Calibre there either. Also, that way I have to manually sync all my devices, which is supremely annoying.

Anyone know a solution for my problem?

Zelda