| /sendgrid |
port 25 経由だけか
gmail.com. 2281 IN MX 10 alt1.gmail-smtp-in.l.google.com. gmail.com. 2281 IN MX 5 gmail-smtp-in.l.google.com. gmail.com. 2281 IN MX 30 alt3.gmail-smtp-in.l.google.com. gmail.com. 2281 IN MX 20 alt2.gmail-smtp-in.l.google.com. gmail.com. 2281 IN MX 40 alt4.gmail-smtp-in.l.google.com. gmail-smtp-in.l.google.com. 300 IN A 74.125.23.26 alt1.gmail-smtp-in.l.google.com. 300 IN A 142.250.141.26
26.97.177.108.in-addr.arpa. 14400 IN PTR tm-in-f26.1e100.net.
$ telnet gmail-smtp-in.l.google.com 25 Trying 74.125.23.27... Connected to gmail-smtp-in.l.google.com. Escape character is '^]'. 220 mx.google.com ESMTP r12-20020a6560cc000000b005c6cc61e9bcsi4405301pgv.359 - gsmtp helo wk.qmail.jp 250 mx.google.com at your service stat^Hrttls 502-5.5.1 Unrecognized command. For more information, go to 502 5.5.1 https://support.google.com/a/answer/3221692 r12-20020a6560cc000000b005c6cc61e9bcsi4405301pgv.359 - gsmtp starttls 220 2.0.0 Ready to start TLS Connection closed by foreign host.
1. whois
$ telnet 108.177.97.26 25 Trying 108.177.97.26... Connected to 108.177.97.26. Escape character is '^]'. 220 mx.google.com ESMTP d11-20020aa78e4b000000b006d08892e9fbsi355128pfr.123 - gsmtp starttls 503-5.5.1 EHLO/HELO first. A mail transaction protocol command was issued out of 503-5.5.1 sequence. For more information, go to 503 5.5.1 https://support.google.com/mail/?p=helo d11-20020aa78e4b000000b006d08892e9fbsi355128pfr.123 - gsmtp helo wk.qmail.jp 250 mx.google.com at your service starttls 220 2.0.0 Ready to start TLS
2. history
$ openssl s_client -connect smtp.gmail.com:587 -starttls smtp CONNECTED(00000003)
$ openssl s_client -connect gmail-smtp-in.l.google.com:587 -starttls smtp xxx