netcat is a very useful tool, but for SMTP testing you might enjoy "swaks": http://web.archive.org/web/20180304190350/https://debian-administration.org/article

netcat is a very useful tool, but for SMTP testing you might enjoy "swaks": http://web.archive.org/web/20180304190350/https://debian-administration.org/article/633/Testing_SMTP_servers_with_SWAKS http://web.archive.org/web/20180304190350/https://debian-adm...