It seems to be a Postfix issue. I don't think you need Mailfix domain to accomplish what you want (send Drupal alerts), so you probably don't need virtual hosts at all. It seems like you want your Drupal server to send out mail messages directly. Thus your Postfix configuration should have mydestination directive include localhost. To verify this:
sudo postconf | grep mydestination
Check if localhost is included in this directive (my guess is it isn't). In case it's not, just reconfigure:
sudo postconf -e "mydestination = you.host.here, localhost, localhost.localdomain"
Then your messages should be sent as expected. HTH