1. SMTP

/現代事情 /proxy /SMTP Email Relay Services

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/

spam/対策

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)

/example

/Gmail example