Converting without saving

Hi All

I have a library of mixed formats, epub, mobi and others, which I manage with Calibre for various reading devices.
Now when I send, say, an epub book to my kindle, Calibre will auto convert this to mobi format before sending it. All well and good. But now I have two copies of the book in the database; the original epub and the converted mobi. How do I prevent Calibre from saving newly converted files to the database?
I would rather have just the one original file in the database and autoconvert it every time I use it.

Cheers

Tom