I think you'll find a way to configure for this under your Mail-->Accounts preferences for Personas. There's a setting for Set the "Reply-to" field of email messages to:
Thanks for your response.
An exemple for our user:
firstname.lastname@example.org as a reply-to: email@example.com
This is really appreciate if myuser was not available, because the external email was reply to the service of myuser, and not personally.
But, when we use the mail for internal request, on the @mydomain.com domain, we want for the reply-to: firstname.lastname@example.org
Any idea? Many thanks and best regards.