CNAME: Mail address gets rewritten

Question

I have a problem with receiving mails. I do not know if my server or the server of the sender is wrong configured.

I have a domain daniel-marschall.de which has following DNS RR:

daniel-marschall.de.    86400   IN  CNAME   viathinksoft.com.

The domain viathinksoft.com has the MX RR to "mail.viathinksoft.com":

viathinksoft.com.   85651   IN  MX  0 mail.viathinksoft.com.

mail.viathinksoft.com runs Postfix, Cyrus and Debian Linux.

When I send an email to webmaster (at) daniel-marschall.de e.g. using GoogleMail, the user "Daniel Marschall" at our system receives the email, which is correct.

But when the server "startcom.org" sends an email to webmaster (at) daniel-marschall.de , the email gets "hard redirected" to webmaster (at) viathinksoft.com which is a different mail account.

The relevant parts of the email headers are:

Received: from viathinksoft.de ([unix socket])
     by viathinksoft.de (Cyrus v2.2.13-Debian-2.2.13-19+squeeze3) with LMTPA;
     Tue, 06 Nov 2012 02:15:02 +0100
Received: by viathinksoft.de (Postfix, from userid 5001)
    id AD83016CE112; Tue,  6 Nov 2012 02:15:01 +0100 (CET)
Received: from mta1.startcom.org (apache-7.startcom.org [192.116.242.7])
    (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits))
    (No client certificate requested)
    by viathinksoft.de (Postfix) with ESMTPS id 90F5116CE107
    for <webmaster (at) viathinksoft.com>; Tue,  6 Nov 2012 02:14:55 +0100 (CET)
Received: from localhost (com.internal.startcom.org [192.168.0.180])
    by mta1.startcom.org (8.13.8/8.13.8) with ESMTP id qA61EbIS010233
    for <webmaster (at) daniel-marschall.de>; Tue, 6 Nov 2012 03:14:37 +0200
From: ...
To: webmaster (at) viathinksoft.com (<-- WRONG!!!)

As you can see, the email was initially intended for "webmaster (at) daniel-marschall.de", but now it has the field "To: webmaster (at) viathinksoft.com", so my MTA dropped the mail to the owner of viathinksoft.com .

I think the sender's MTA thinks that "CNAME" RR means that the address needs to be rewritten, but as far as I know, CNAME is only a soft redirect on DNS basis and should not force the MTA to rewrite the domain name. Or am I wrong?

PS: The log entries are showing that my server already has the wrong address when the mail is incoming:

$ cat /var/log/mail.log | grep '90F5116CE107'
Nov  6 02:15:00 viathinksoft postfix/smtpd[18184]: 90F5116CE107: client=apache-7.startcom.org[192.116.242.7]
Nov  6 02:15:00 viathinksoft postfix/cleanup[19928]: 90F5116CE107: message-id=<c76cb951ea724270c94ff1b0e8d4de4e (at) startcom.org>
Nov  6 02:15:00 viathinksoft postfix/qmgr[9843]: 90F5116CE107: from=<certmaster (at) startcom.org>, size=1583, nrcpt=1 (queue active)
Nov  6 02:15:01 viathinksoft postfix/pipe[20419]: 90F5116CE107: to=<info (at) viathinksoft.de>, orig_to=<webmaster (at) viathinksoft.com>, relay=spamassassin, delay=6.2, delays=5.5/0/0/0.76, dsn=2.0.0, status=sent (delivered via spamassassin service)
Nov  6 02:15:01 viathinksoft postfix/qmgr[9843]: 90F5116CE107: removed

Answer 1

As the comments indicated, you can't have your MX as a CNAME. You can detect problems like this by using a site like intoDNS. Here is the report for your domain -- I see from it that you've already fixed the problem.