dkim - Emails is marked as spam

Hi We have set up the DKIM in our mail server using sendmail and configured our DNS record also. Still our mails are marked as spam. Also when we sent the mail from outlook the domainkey authentication is pass whereas when the mail is sent from PHP ( PEAR package MAIL ), Yahoo is marking authentication as fail

When sent via PHP, Yahoo is adding the following header Authentication-Results: mta150.mail.in.yahoo.com from=mydomain.com; domainkeys=fail (bad sig); from=mydomain.com; dkim=permerror (bad sig)

When sent from Outlook, the header is Authentication-Results: mta175.mail.in.yahoo.com from=mydomain.com; domainkeys=pass (ok); from=mydomain.com; dkim=pass (ok) which indicates that Domain Key authentication has passed

I examined the headers when sent from PHP and compared it with what Outlook is adding and did not see much difference. While sending it from PHP I am adding the headers From, To, Message-ID, Mime-Version, Date, Subject,Content-Type and Outlook is also having the same,

What else could be wrong in the mail while sending mail via PHP because of which authentication is failing

PHP does not use the local MTA on the server, it connects to the SMTP server ( SendMail ) which is on a different machine. The PEAR MAIL uses the socket API to connect to the SMTP server and uses the SMTP protocol, I do not think the mail is signed at the WebServer, it is only signed on the SMTP server.

We use SendMail on the Mail Server and both Outlook and PHP uses the same SMTP server with same login and password and same authentication mechanism

That is the reason why I am confused as to what could be the problem. Is there some standard way a mail should be constructed which can effect the signing of the mail, which Outlook is doing correctly but when done via PHP, the mail is constructed differently, at least we have verified that the headers are the same.

Below mentioned are the headers yahoo received when I sent mail to the that yahoo account.
This mail is sent by PHP Zend framework.

Earlier I sent mail using PHP PEAR package, At that time
Yahoo was adding the following header
Authentication-Results: mta150.mail.in.yahoo[dot]com from=mydomain[dot]com;

domainkeys=fail (bad sig); from=mydomain[dot]com; dkim=permerror (bad sig).

But Now when I use PHP Zend framework to send mail, dkim is set to pass(ok ) but domainkeys=fail.
Header is as follows
Authentication-Results: mta153.mail.in.yahoo[dot]com from=mydomain[dot]com; domainkeys=fail (bad sig); from=mydomain[dot]com;
dkim=pass (ok).

So what does dominkeys=fail ( bad sig ) mean ? and how is it different from dkim=pass (ok )

X-Apparently-To       :  ravi.cybertechie@yahoo[dot]com via 203.104.18.180; Wed, 
                         28 Oct 2009 18:03:13 +0530
Return-Path           :  
X-YMailISG            :  YISh33wWLDtwDNH0f_08ONu7pG42J7wuD.SOtD4sPZcX_x7EOghleA5_M5U7vRhdf7ZIIIo0yVtS35PCaQRAY2w8Naq8c3wgufoQrWvJyQt6jIH0BRkuwgUERH7N7FHGLQpnCT0A9ig_TvthXoXJ3Vc.mQFe.OPT4CE6Vz1kgCsBEVxb.M2Zcr4BYK27XARjHg7hCsNDFi_ddp.Y.s6njtr7fnbQl0SD0gCcK2qNVxxKNhoukmuvpE4BPA_EyVEgNqJGjcBYh_GYqsapic4zKDWhcb4nOBAAzDR0XtydFiX3nj_TqDI0v_zNwuUK0K53tYdnI3HCSg--
X-Originating-IP      :  [my_smtpserver_ipaddress]
Authentication-Results: mta153.mail.in.yahoo[dot]com from=mydomain[dot]com; 
                        domainkeys=fail (bad sig); from=mydomain[dot]com;
                        dkim=pass (ok)
Received              : from my_smtpserver_ipaddress (EHLO my_smtpserver_hostname)
                        (my_domain_ip_address) by mta153.mail.in.yahoo[dot]com with SMTP; Wed,

                        28 Oct 2009 18:03:13 +0530
Received-SPF          : pass (my_smtpserver_hostname: authenticated connection)
                        receiver=my_smtpserver_hostname; client-ip=my_mailclient_ipaddress;
                        helo=localhost; envelope-from=admin@mydomain[dot]com;
                        x-software=spfmilter 0.97 
                        http://www.acme[dot]com/software/spfmilter/ with libspf2-1.0.0;
Received              : from localhost ([my_mailclient_ipaddress]) (authenticated bits=0) 
                        by my_smtpserver_hostname (8.14.2/8.14.2) 
                        with ESMTP id n9SCX6ml017820 for ; 
                        Wed, 28 Oct 2009 08:33:08 -0400

X-DKIM                : Sendmail DKIM Filter v2.5.1
                        my_smtpserver_hostname n9SCX6ml017820
DKIM-Signature        : v=1; a=rsa-sha256; c=simple/simple; d=mydomain[dot]com; s=mydomain;
                        t=1256733190; bh=oBeBoCqUcTrV/4cgaMK58OrPkchqCMzB2Vax aRRph6g=;
                        h=From:To:Subject:Date:Content-Type:MIME-Version;
                        b=faLHQ jAakBDkxbz8xhGmfi2ZfBHXUBFOWTZs9BAfaksB8lJghho6YmUPKIY5oMJMsCkh/QLQ 8RXD8dUPf04X6A==
X-DomainKeys          : Sendmail DomainKeys Filter v1.0.2 
                        my_smtpserver_hostname n9SCX6ml017820
DomainKey-Signature   : a=rsa-sha1; s=mydomain; d=mydomain[dot]com; c=simple; q=dns;                        b=3Wazgw8//NzW95FYyQGZdXDuc4mLkp6+OdOIv224+aC0imfrA3GoeSEIt0UTyaT9d ygfHc6OKuxy7cz0/a3FnA==
Message-Id            : 

From                  : mydomain Admin  Add sender to Contacts
To                    : Recipient 
Subject               : Mail from zend
Date                  : Wed, 28 Oct 2009 12:33:05 +0000
Content-Type          : multipart/alternative; charset="iso-8859-1";
                        boundary="=_2f9550c6ce7fb246f62d84f1221854d7"
MIME-Version         : 1.0
Content-Length       : 4918