interactive SMTP command using telnet via a shell script

telnet fails for a variety of reasons. First up, if you strace it the relevant errors are telnet wanting a TTY on standard input, of which there is none (because of the redirect) so telnet fails.

$ strace telnet mx.example.edu 25 < input
...
ioctl(0, SNDCTL_TMR_STOP or TCSETSW, {B0 -opost -isig -icanon -echo ...}) = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(0, SNDCTL_TMR_START or TCSETS, {B0 -opost -isig -icanon -echo ...}) = -1 ENOTTY (Inappropriate ioctl for device)

If instead nc or netcat is used, the non-interactive send may still be a problem; either too much is sent in one go (which will confuse the remote server) or the server will object to too much data too quickly; SMTP is an interactive conversation. If one side is blasting away the other may delay or reject that (sending too quickly can be an indication of a spammer). If your nc supports --delay and the remote mail server is forgiving you may be able to to send.

$ strace -s 80 nc --crlf --delay 1 mx.example.edu 25 < input
...
recvfrom(3, "220 mx.example.edu ESMTP OpenSMTPD\r\n", 8192, ...
...
sendto(3, "HELO client.example.edu\r\nmail from: [email protected]\r\n...
...
shutdown(3, SHUT_WR)                    = 0
...
recvfrom(3, "500 5.5.1 Invalid command: Pipelining not supported\r\n",

OpenSMTPD here rejects the send; Postfix on a different server (not shown here) allows the above send.

A SMTP client that speaks the SMTP protocol really should be used instead; this could be kludged together with something like expect https://stackoverflow.com/questions/12320592/telnet-smtp-with-expect-or-shell-script or otherwise there are various SMTP libraries in various languages that will support TLS, SMTP AUTH, handle errors, and so forth. The shell is not a good choice here...unless you use the TCP features of a shell like ZSH, but there's probably better things to do than to "not invented here" yet another SMTP client...