MS outlook problem

Our Medical Billing Manager complained me that she had faced very strange problem, no mails going out to yahoo ID from MS outlook (Gmail ID configured on it), but using the webbased panel of Gmail mails are sucessfully going through to same yahoo ID. The error we are getting while sending mails from MS outlook (Gmail ID configured on it) was:

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Technical details of permanent failure:Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 554 554 Message not allowed - [320] (state 18).
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I started finding the solution of this problem using yahoo and google blogs and finally one strange IT person on a technical forum shared this info with me:

=======Thank you for writing to Yahoo! Mail.
We thank you for your message. This error message indicates that your email was not accepted because the mail in question may contain characteristics that Yahoo! Mail will not accept for policy reasons. For instance, it is against Yahoo! Mail's policy to accept messages with malicious content or manipulated header information, such as a falsified origination point, altered sending date, or forged email address.
If you are the administrator for the message content or mailing policies, we ask that you review outgoing messages for potentially objectionable content or practices. Please know that Yahoo! is unable to disclose specific information regarding our filtering practices, however do publish recommended guidelines at the following site:
http://help.yahoo.com/l/us/yahoo/mail/postmaster/basics/postmaster-15.html

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After going through all this i first checked the date and time of that PC, found that it was changed to the future date and time and because of this our Medical Billing Manager not able to send mails to yahoo IDs using MS outlook.

Now, Problem RESOLVED.