There are two cases.

Case 1: message does not signed.
SMTP session:
================================8<=======================
[***@mailrelay.ourdomain.com /]# telnet 0 25
Trying 0.0.0.0...
Connected to 0.
Escape character is '^]'.
220 mailrelay.ourdomain.com ESMTP Postfix
EHLO localhost
250-mailrelay.ourdomain.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
MAIL FROM: <***@ourdomain.com>
250 2.1.0 Ok
RCPT TO: <***@gmail.com>
250 2.1.5 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
From: <***@ourdomain.com>
To: <***@gmail.com>

.
250 2.0.0 Ok: queued as 427862C8819
================================8<=======================

Postfix log:
================================8<=======================
2012-08-17T17:08:29.360923+04:00 mailrelay postfix/smtpd[22226]:
connect from localhost.localdomain[127.0.0.1]
2012-08-17T17:08:41.274738+04:00 mailrelay postfix/smtpd[22226]:
427862C8819: client=localhost.localdomain[127.0.0.1]
2012-08-17T17:08:51.217937+04:00 mailrelay postfix/cleanup[21928]:
427862C8819: message-id=<***@mailrelay.ourdomain.com>
2012-08-17T17:08:51.218055+04:00 mailrelay dkim-filter[1425]:
(unknown-jobid): no sender header found; accepting
2012-08-17T17:08:51.222046+04:00 mailrelay postfix/qmgr[19619]:
427862C8819: from=<***@ourdomain.com>, size=204, nrcpt=1 (queue
active)
2012-08-17T17:08:51.511028+04:00 mailrelay postfix/smtp[22194]:
427862C8819: to=<***@gmail.com>,
relay=gmail-smtp-in.l.google.com[173.194.70.27]:25, delay=19,
delays=19/0/0.05/0.24, dsn=2.0.0, status=sent (250 2.0.0 OK 4145308131
l8si01973873odq.99)
2012-08-17T17:08:51.511150+04:00 mailrelay postfix/qmgr[19619]:
427862C8819: removed
================================8<=======================

Case 2: message signed successfully.
Sic! I just changes