[Exim] identifying real sender

Thanks for the help earlier, but I don't think I was asking the right
question. What I would like to do is have a special alias director that
will accept a class name like ss01-cse-498-2@??? and then do a mysql
lookup for all of the students matching that class and look up the
professor's name in another table to see if he/she is authorized to mail
to that class. I've got part of that working with the following

special_director:
  condition = ${if match{$local_part}{....-.+-.+-.+}{true}{false}}
  driver = aliasfile
  search_type = mysql
  query = Select student from clist left join author using (course) 
          where sem = '${quote_mysql:${extract{1}{-}{$local_part}}}'
          and clist.sub = '${quote_mysql:${extract{2}{-}{$local_part}}}'
          and clist.course =
'${quote_mysql:${extract{3}{-}{$local_part}}}'              and section =
'${quote_mysql:${extract{4}{-}{$local_part}}}'
          and username = '${extract{1}{@}{$sender_address}}' 

My problem is the $sender_address can be easily faked. Is there any way
to check the validity of $sender_address in Exim? I have searched the
manual but either I'm not understanding something or I can't find it.

I tried $authenticated_id with this in the authentication of the config

fixed_plain:
    driver = plaintext
    public_name = PLAIN
    server_condition = \
        ${if crypteq {$3}{${lookup mysql {MYSQL_PLAIN}{$value}{no}}}{yes}{no}}
    server_set_id =
$2                                                                                                                        

Thank you

AKB