I like this one.

  $ cat /usr/local/bin/awkmail
  #!/bin/gawk -f

  BEGIN { smtp="/inet/tcp/0/smtp.yourco.com/25";
  ORS="\r\n"; r=ARGV[1]; s=ARGV[2]; sbj=ARGV[3]; # /bin/awkmail to from subj < in

  print "helo " ENVIRON["HOSTNAME"]        |& smtp;  smtp |& getline j; print j
  print "mail from:" s                     |& smtp;  smtp |& getline j; print j
  if(match(r, ","))
  {
   split(r, z, ",")
   for(y in z) { print "rcpt to:" z[y]     |& smtp;  smtp |& getline j; print j }
  }
  else { print "rcpt to:" r                |& smtp;  smtp |& getline j; print j }
  print "data"                             |& smtp;  smtp |& getline j; print j

  print "From:" s                          |& smtp;  ARGV[2] = ""   # not a file
  print "To:" r                            |& smtp;  ARGV[1] = ""   # not a file
  if(length(sbj)) { print "Subject: " sbj  |& smtp;  ARGV[3] = "" } # not a file
  print ""                                 |& smtp

  while(getline > 0) print                 |& smtp

  print "."                                |& smtp;  smtp |& getline j; print j
  print "quit"                             |& smtp;  smtp |& getline j; print j

  close(smtp) } # /inet/protocol/local-port/remote-host/remote-port