outlook sending messages to people from my Address book

i dont understand here what is going on i am running outlook on it and i also have a server in my office which is NT 4.0. But for some what odd reason e-mails are being send from my account which me knowing and the address are in my adress book. first i thought it could be some sort of virus who is doing that but i have updated my anti virus and i have also ran a full scan on my PC and the server but i and still getting the same problem an example is below:
 



-----Original Message-----
From: System Administrator []
Sent: 29 July 2004 20:20
To:
Subject: Undeliverable: [VIRUS REMOVED] Returned mail: Data format error

Your message

  To:      
  Subject: [VIRUS REMOVED] Returned mail: Data format error
  Sent:    Thu, 29 Jul 2004 15:20:06 -0400

did not reach the following recipient(s):


    The recipient name is not recognized
      The MTS-ID of the original message is: c=us;a=
;p=amp;l=US194MX000407291920P6CF4H51
    MSEXCH:IMS:AMP:TE1:US194MX00 0 (000C05A6) Unknown Recipient
-----
Subject: [VIRUS REMOVED] Returned mail: Data format error
Date: Thu, 29 Jul 2004 15:20:06 -0400
X-Mailer: Internet Mail Service (5.5.2657.72)
X-MS-Embedded-Report:

The message was not delivered due to the following reason(s):

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