I have problem with mail forwarding from old mailserver:

- mail1@oldserver.com (old webserver) is configured to forward all incoming mails to mail2@newserver.com

if I'm sending mail to mail1@oldserver.com from another server (gmail.com and others) forward works fine

for better understanding:
mail@gmail.com --send--> mail1@oldserver.com --forward--> mail2@newserver.com (works fine)

but when sending mails from new server to old server, forwarding back to new server fails: 
mail.log reports - Sender address rejected: not logged in;

for better understanding:
mail2@newserver.com --mail--> mail1@oldserver.com --forward--> mail2@newserver.com (fails)

Is there any other solution except
mynetworks = oldserver
in postfix configuration?

I have iRedmail 6.0.1 with OpenLDAP backend.

Thanks!