Who’s your SMTP daddy?
In a hotel in Beijing, using their wifi in the lobby. Everything goes fine until Noam tells me my email headers are weird.
Received: (qmail 9613 invoked from network); 19 Nov 2008 13:26:43 -0000
Received: from mail.hsia.com.cn (HELO hsia.com.cn) (18.104.22.168)
by 0 with SMTP; 19 Nov 2008 13:26:43 -0000
Received: from FBH.hsia.com.cn ([22.214.171.124])
by hsia.com.cn (8.13.1/8.13.1) with ESMTP id mAJDTJlY005475;
Wed, 19 Nov 2008 21:29:20 +0800
Received: from beat.local (unknown [172.31.8.65])
by FBH.hsia.com.cn (Postfix) with ESMTP id 8AFEB520B0;
Wed, 19 Nov 2008 21:13:54 +0800 (CST)
Clearly I’m sending through another SMTP server, who goes as far as mangling my ‘Return-Path’ address header.
Only I’m not. My SMTP server is set (as always) to the corporate SMTP who is accessible through the VPN, in an encrypted connection that should not allow anyone to change fields. Just in case, I check it again. Yup, the SMTP server is there. So what’s up?
A quick investigation shows the following: The hotel’s network blocks my VPN (as some of them do) but happily resolves any unresolvable host name (such as my SMTP server’s hostname). This is resolved to a catch-all server that proxies everything. Transparently. (well, almost)
Lesson learned. Changed the hostname to the IP, and will soon switch to SSL based SMTP who will authenticate the server. In the meanwhile – be careful from helpful Beijing wifi providers who are only too happy to forward your mail on! (with some changes, of course).