How to properly configure sender address substitution in Postfix?

Question

Email

There is a mail server on postfix in the local network and the domain mydomain.local
Mail goes within the local network. There is a user [email protected] who is allowed to send emails to the Internet. Sending mail through the hosting provider's relay is configured and the /etc/postfix/generic file specifies the correspondence of the local address to the mail address located on the hosting. The contents of the file /etc/postfix/generic:
[email protected] [email protected]
Mail is normally sent to the Internet. But if the user sends a letter from the mailbox [email protected] to the internal user [email protected], then the letter comes to him already with the name [email protected] and the user [email protected] can no longer respond, because it is not allowed to send to the internet.
What needs to be done so that for internal users there is no address spoofing, and the substitution is only if the user sends [email protected] to the Internet? I kindly ask for help in solving this problem.