perl -e '$|++;open $f,$ARGV[0];while($l=<$f>){print $l;sleep 1;}' "file.txt" | tee /dev/tty | telnet -8 smtp.site.ru 25

AUTH LOGIN
логин
пароль
MAIL FROM:<...>
RCPT TO:<...>
DATA
From: ... <...>
To: ... <...>
Subject: Test message
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Полужирный текст
.
quit