allow senders of my domain via authentication only

[univocal]

Dear All,

can any body help me in implementing my scenario.

i use postfix, to send emails of my users.

i have allowed permit mynetworks of ( 10.0.0.0/8,
192.168.15.1/24....etc), So, All of my users are able to send emails
from any domain to any domain , i have implemented MailScanner as
well, which detect spam and stops.

All of our users are able to relay emails via our MailServer. I have
also a domain mydomain.net, most of our users uses fake sender email
addresses of mydomain.net, which i want to stop.

I want to implement authentication for only a specific domain
mydomain.net. the users of mydomain.net net first get authentication
and then be able send emails.

My all remaining users using their owndomain must remain able to send
emails as they are doing now.

Can anyone help me in this regards.

Best Regards
James