MX entry in DNS

[Thomas Ludwig]

I'm afraid, it's a stupid question... but:

I know that the MX (Mail eXchange) entry points to mail-servers, e.g.
servers offering smtp.
But I don't know what the use of these entries is. Which program(s)
use(s) it?
It least our smtp-server doesn't need the MX entry (but needs a 'mailhost'-entry).

Could someone please enlighten me?

Thanks
-- 
Thomas Ludwig
Institute of Mineralogy
University of Heidelberg, Germany