I have virtualmin and settings on virtualhost: mailname and hosts mydomain.com
I have all my emails accounts on gmail e.g. email@example.com
However when sending emails in Joomla (PHP Mail setting) I have in mail.log
8BB689FE64: to=<firstname.lastname@example.org>, relay=local, delay=0.05, delays=0.03/0/0/0.01, dsn=5.1.1, status=bounced (unknown user: "support")
It treats mydomain.com as local, this domain is on gmail.
When sending to other than mydomain.com it send fine.