trusted routing problem

Hello,

following problem:

we have an old mailserver called xyz.local this server (old smtp to host
novell gateway)

the gateway sends the email over our central smtp-gateway (running postfix)

The Email adresses have the form: user%xyz.local@ourdomain.com

Previous we set allow_untrusted_routing to yes so emails
like user@xyz.local@ourdomin.com reaches our postfix smtp-gateway and
were forwarded to the old novel gateway.

But with this configuration our Postfix smtp-Gateway could be used as
open relay (e.g. user@somewhere@ourdomin.com would be send to
user@somewhere)

So we set allow_untrusted_routing to no. Now
user%xyz.local@ourdomain.com is also denied.

Unfortunately we could not change the address
user%xyz.local@ourdomain.com because this adresses where used since
years from our CEO and he don't want to change his Email address.

So is it possible to allow EMail adresses like
user%xyz.local@ourdomain.com and disallow user@somewhere@ourdomin.com???

Thanks
Dida