smtp-test.sh

#!/bin/bash

read_response() {
    while read -u9 LINE; do
        declare more_response=${LINE:3:1}
        echo $LINE
        [ "$more_response" != '-' ] && break
    done
}

exec 9<>/dev/tcp/mx1.mail.yahoo.co.jp/25

read_response

echo -ne "EHLO localhost\r\n" >&9

read_response

echo -ne "MAIL FROM: <user@localhost>\r\n" >&9

read_response

echo -ne "RCPT TO: <someone@example.com>\r\n" >&9

read_response

# Result
#
# 220 mta508.mail.kks.yahoo.co.jp ESMTP YSmtp service ready
# 250-mta508.mail.kks.yahoo.co.jp
# 250-8BITMIME
# 250-SIZE 41943040
# 250 PIPELINING
# 250 sender <user@localhost> ok
# 550 relaying denied for <someone@example.com>