postfix uses hostname, not domainname
 

When i send e-mail, the emails get sent from "user@machine.domain.com" instead of "user@domain.com"

The myorigin parameter is set to $mydomain, which is set to my "domain.com"

Any suggestions? I searched but didn't find any clear answers on this.

Try mydestination=$mydomain
instead of mydestination=$myhostname