[exim] transport rule

When writing a transport rule in exim v4.41, to deliver an email
addressed to "randomlocalpart123@???" into a mbx file
"/home/$subdomain/mail/INBOX", what is the correct variable for
"$subdomain"?

custom_delivery:
   driver = appendfile
    file = /home/$subdomain/mail/INBOX
   mbx_format
   delivery_date_add
   envelope_to_add
   return_path_add
# group = mail
   mode = 0600

However,
1) there is no such 'user' as 'randomlocalpart123', it is a randomly
     generated string.
2) there is a username of "subdomain"
3) "subdomain" is NOT the hostname of the server.

Any help greatly appreciated.


Regards,

- - - - - - - - - - - - - - - -

Ken Jackson.
Unix & Email Systems Administrator,