Most recieving mail systems will reject an email with a ‘from’ of [email protected] when the email is sent from the mydomain.com system. This is to prevent mail relaying, a form of spam.
You need to make sure that your ‘from’ email address matches your domain. The actual email address (like [email protected] for mydomain.com) doesn’t need to exist, but the mydomain.com in the email must match the mydomain.com domain name of the sending mail server.
You can always set the ‘reply-to’ of an email as [email protected], and that mail will be delivered through yourdomain.com mail server. That allows someone to reply to your gmail account.