Here is a script that I use to send SMTP mail, via the gawk networking extensions. I have a few different versions, but this is the most basic:

    #!/bin/gawk -f

    BEGIN { smtp="/inet/tcp/0/smtp.yourhost.com/25";
    ORS="\r\n"; r=ARGV[1]; s=ARGV[2]; sbj=ARGV[3]; # /usr/local/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

This allows me to bypass the local MTA (if present). The message ID is also returned, which can be useful to log.