[TUHS] where is "the true arpanet mailer"?

I am reading the delivermail (later known as postbox and then sendmail) code from 4.0BSD and from sccs history from June 1980. Its arpa-mailer(8) manual says it just spools the letter and actual delivery will be performed by the ARPANET mailer daemon and refers to mailer(ARPA) manual. The arpa.c code says "is stuck away in the outgoing arpanet mail queue for delivery by the true arpanet mailer." Where is this true arpanet mailer? I am guessing it periodically looks in /usr/spool/netmail/ and delivers the messages using FTP and RFC458. Where is this mailer(ARPA) manual? Where is the ftp server code used for the incoming mail? (Example code mail-dm.c is provided for the ftp server to "handle the MAIL <user> command over the command connection.") Also where is the uucp-mailer(8) manpage referenced in delivermail(8)? Jeremy C. Reed echo 'EhZ[h ^jjf0%%h[[Zc[Z_W$d[j%Xeeai%ZW[ced#]dk#f[d]k_d%' | \ tr '#-~' '\-.-{'