Why We Can Send to Gmail in China
EDIT: format<p>The post only tells part of the story. There are at least two places that GFW can (and does!) block email traffic as long as the traffic goes through GFW:<p>1. By DNS poisoning your domain name. There&#x27;s nothing special, when GFW decide to DNS poisoning your domain, all query types will be poisoned, including your MX record.<p>2. By TCP reset your SMTP connection to MTA (or forge reply from the other end) if the sender or recipient is something special.<p>For #1, this is happening to my domain name yegle.net. This could be demonstrated via a DNS query sent from outside of China to <i></i>any<i></i> servers (even it&#x27;s not a DNS server) in China (see examples in the end of this comment).<p>For #2, this is happening to my gmail account. Try connect to an MTA in China from outside of China, as soon as you type &quot;MAIL FROM: MY_EMAIL_ADDRESS&quot;, you&#x27;ll get a TCP reset. (see examples in the end of this comment). If you are sending email from China to my email address, you&#x27;ll get a forged reply saying the email address doesn&#x27;t exist (again see example in the end of this comment).<p>In order to make sure you can send&#x2F;receive email from&#x2F;to China, you need to make sure the sender and receiver&#x27;s email service support [StartTLS](<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Opportunistic_TLS" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Opportunistic_TLS</a>).<p><pre><code>    &#x2F;&#x2F; TEST FROM OUTSIDE OF CHINA
    $ dig MX yegle.net @54.222.60.218

    ; &lt;&lt;&gt;&gt; DiG 9.9.5-3ubuntu0.8-Ubuntu &lt;&lt;&gt;&gt; MX yegle.net @54.222.60.218
    ;; global options: +cmd
    ;; Got answer:
    ;; -&gt;&gt;HEADER&lt;&lt;- opcode: QUERY, status: NOERROR, id: 9506
    ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

    ;; QUESTION SECTION:
    ;yegle.net.                     IN      MX

    ;; ANSWER SECTION:
    yegle.net.              2654    IN      A       8.7.198.45

    ;; Query time: 176 msec
    ;; SERVER: 54.222.60.218#53(54.222.60.218)
    ;; WHEN: Mon Sep 19 13:13:17 PDT 2016
    ;; MSG SIZE  rcvd: 43

    &#x2F;&#x2F; TEST FROM OUTSIDE OF CHINA
    $ dig MX yegle.net @8.8.8.8

    ; &lt;&lt;&gt;&gt; DiG 9.9.5-3ubuntu0.8-Ubuntu &lt;&lt;&gt;&gt; MX yegle.net @8.8.8.8
    ;; global options: +cmd
    ;; Got answer:
    ;; -&gt;&gt;HEADER&lt;&lt;- opcode: QUERY, status: NOERROR, id: 17621
    ;; flags: qr rd ra ad; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 1

    ;; OPT PSEUDOSECTION:
    ; EDNS: version: 0, flags:; udp: 512
    ;; QUESTION SECTION:
    ;yegle.net.                     IN      MX

    ;; ANSWER SECTION:
    yegle.net.              299     IN      MX      10 aspmx.l.google.com.
    yegle.net.              299     IN      MX      20 alt1.aspmx.l.google.com.
    yegle.net.              299     IN      MX      30 alt2.aspmx.l.google.com.
    yegle.net.              299     IN      MX      40 aspmx2.googlemail.com.
    yegle.net.              299     IN      MX      50 aspmx3.googlemail.com.

    ;; Query time: 24 msec
    ;; SERVER: 8.8.8.8#53(8.8.8.8)
    ;; WHEN: Mon Sep 19 13:13:27 PDT 2016
    ;; MSG SIZE  rcvd: 171


    &#x2F;&#x2F; TEST FROM OUTSIDE OF CHINA
    $ telnet mail.kingsoft.com 25
    Trying 219.141.176.248...
    Connected to telecom.mail.kingsoft.com.
    Escape character is &#x27;^]&#x27;.
    220 mail.kingsoft.com ESMTP
    EHLO gmail.com
    250-mail.kingsoft.com
    250-8BITMIME
    250 SIZE 52428800
    MAIL FROM: cnyegle-AT-gmail-com
    Connection closed by foreign host.

    &#x2F;&#x2F; TEST FROM INSIDE OF CHINA
    $ telnet aspmx.l.google.com 25
    Trying 209.85.225.27...
    Connected to aspmx.l.google.com.
    Escape character is &#x27;^]&#x27;.
    220 mx.google.com ESMTP u6si11379881igw.58
    EHLO yegle.net
    250-mx.google.com at your service, [183.151.34.162]
    250-SIZE 35882577
    250-8BITMIME
    250-STARTTLS
    250 ENHANCEDSTATUSCODES
    MAIL FROM:&lt;mail@example.com&gt;
    250 2.1.0 OK u6si11379881igw.58
    RCPT TO:&lt;cnyegle-AT-gmail-com&gt;
    551 User not local; please try &lt;forward-path&gt;
    Connection closed by foreign host.</code></pre>