Seems not that good to just remove server hostname in "mydestination=". Please try this:
*) Change your server hostname to one different from your mail domain name. e.g. mx.mydomain.org.
*) Then add your server domain name, mx.mydomain.org, in Postfix "mydestination=", and update "myhostname=mx.mydomain.org".
*) Restart Postfix and try again.
