I have looked into the email function of calibre. Turns out that it sends an automatic email once the recipe is done. It will use the result of the recipe. Unfortunately, calibre's recipe system is being kind of tricked out by my recipe - it only knows of the converted epub, and the other files are downloaded behind it's back... So, there is no way the recipe system could trigger an email with files it does not know about. Even if I included an option not to download the epub version, the recipe system would still insist in building some dummy nonsense epub, which would then be sent by email...
You should however be able to send the mobi file by manual selection. Just be sure to have erased the 'epub' option and included the 'mobi' option in calibre's email configurations.