bash and telnet to test an email

Try doing this :

[[ $4 ]] || {
    printf "Usage\n\t$0 <domain> <email> <from_email> <rcpt_email>\n"
    exit 1
}
{
    sleep 1
    echo "helo $2"
    sleep 0.5
    echo "mail from:<$3>"
    sleep 0.5
    echo "rcpt to:<$4>"
    echo
} | telnet $1 25 |
    grep -q "Unknown user" &&
    echo "Invalid email" ||
    echo "Valid email"

Usage :

./script.sh domain email from_email rcpt_email