mstdn.rinsuki.net is one of the many independent Mastodon servers you can use to participate in the fediverse.

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rinsuki

$ 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 google.com/search?btnI&q=RFC+5 xxxxxxxxxxxxxx.xxx - gsmtp

ワロタ

www.google.comRFC 5321 - Google 検索