check_client_access using smtp auth usernames

[Nikolaos Milas]

I have parenthetically asked in another - solved - thread if postfix 
offers the ability to control access to specific mail addresses using as 
a key the authenticated usernames (and got no reply).

So, I am posting this as a new thread, hoping that someone has faced 
this scenario.

The idea is: a user is trying to connect to our SMTP (postfix) to send 
email to onemailaddr...@example.com (which we want to control access 
to). He is authenticated by postfix as userx using SASL over our LDAP 
server (we are using virtual accounts only). Can we setup some 
"smtp_restriction_classes" based on authenticated usernames (for example 
a check_client_access lookup table with entries of the form: "userx      
OK", where userx is a successfully authenticated SMTP username and not 
the sender's mail address username)?

Is there direct or indirect way to accomplish this? Is there a way to 
"retrieve" the SMTP authentication username and process it?

I haven't been able to locate a similar subject in the documentation 
(except of course permit_sasl_authenticated which applies to all SASL 
authenticated users at the same time).

Thanks very much,
Nick