The DOOR.ID file

 Many maildoors now produce (and readers use) a file called Door.Id that
 was developed by Greg Hewgill.  It is intended to give the reader
 information on the capabilities of the door that produced the packet (and
 presumably will process the *.REP produced).  It is a straight text file
 with the following format.  Each line is in the format ControlWord space
 equals space value.

   DOOR = <doorname>
     This is the name of the door that created the QWK packet,
       eg. <doorname> = Tomcat.

   VERSION = <doorversion>
     This is the version number of the door that created the packet,
       eg. <doorversion> = 2.9.

   SYSTEM = <systemtype>
     This is the underlying BBS system type and version,
       eg. <systemtype> = Wildcat 2.55.

   CONTROLNAME = <controlname>
     This is the name to which the reader should send control messages,
       eg. <controlname> = TOMCAT.

   CONTROLTYPE = <controltype>
     This can be one of ADD, DROP, or REQUEST (or others). ADD and DROP are
     pretty obvious (they work as in Markmail), and REQUEST is for use with
     BBS systems that support file attachments.  Try out SLMR with
     CONTROLTYPE = REQUEST and use the Q function.

   RECEIPT
     This flag indicates that the door/BBS is capable of return receipts
     when a message is received.  If the first three letters of the subject
     are RRR, then the door should strip the RRR and set the
     'return-receipt-requested' flag on the corresponding message.