$ telnet -4 gmail-smtp-in.l.google.com 25
Trying xx.xxx.xxx.xx...
Connected to gmail-smtp-in.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP xxxxxxxxxxxxxx.xxx - gsmtp
HELP
214 2.0.0 https://www.google.com/search?btnI&q=RFC+5321 xxxxxxxxxxxxxx.xxx - gsmtp
ワロタ