How to specify default MTA?

i'm trying to use the MAILTO function in crontab on my mybookworld (it's a nas)

I've installed a rather unfamiliar(in think) MTA: esmtp

With mutt(console based email client) it works perfectly. But in crontab there is no reaction(even if i'm using the s开发者_如何学Goame crontab file as on my desktop machine with postfix as MTA). So i think maybe the system doesn't know that esmtp is a MTA that could be used from crontab.

How can i tell crontab what mta to use?

greetings