strange error with mailer daemon

[bortolini]

Hi All,
I have working Zimbra 6.0.0 AG.
Today I receive a rare report for an user:

The user sends one email the last week (April 1st).

from:
user1@mydomain.com
to:
user3@domain1.com
user4@domain2.com
cc:
user2@mydomain.com

We have a bad configuration with our DNS Reverse and the domain2.com didn't accept the message, the log show:

Apr 1 22:35:51 webmail postfix/smtp[16260]: 336682F0BFD: to=<user2@domain2.com>, relay=mx.domain2.com[115.212.164.02]:25, delay=330444, delays=330443/0.02/0.47/0.06, dsn=4.7.1, status=deferred (host mx.domain2..com[115.212.164.02] said: 450 4.7.1 Client host rejected: cannot find your hostname, [194.50.33.158] (in reply to RCPT TO command))

but, when the mailer-daemon send the "Undelivered Mail Returned to Sender" it's sent to user2@mydomain.com (the cc account).

I don't understand why the mailer-daemon send the reply to cc(user2@mydomain.com) user and why it don't reply to from user(user1@mydomain.com).

We use the web client to send emails.

thanks for your help.