1. SMTP
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古い技術について—SMTP現代事情つまみ食い— https://speakerdeck.com/azumakuniyuki/gu-iji-shu-nituite-smtpxian-dai-shi-qing-tumamishi-i
error code : SMTP errors and reply codes https://serversmtp.com/smtp-error/
What's an SMTP port & how to choose the right one https://postmarkapp.com/blog/choosing-the-right-smtp-port#smtp-port-587-the-right-choice-for-nearly-every-situation
1.1. example
$ openssl s_client -connect smtp.spmode.ne.jp:465
openssl s_client -starttls smtp -connect their_smtp_server:587
$ openssl s_client -starttls smtp -connect smtp.spmode.ne.jp:465 CONNECTED(00000003)