How to fix postfix: Sender address rejected: need fully-qualified address (in reply to RCPT TO command))?

$mydomain is used as a default value for many other configuration parameters, but it does not set the sender domain.

Take a look to the $myorigin parameter in /etc/postfix/main.cf

It specifies the domain that appears in mail that is posted on this machine. The default is to use the local machine name, $myhostname, which defaults to the name of the machine.