People who recieve emails from you when you use Microsoft Outlook 2003 can see what client you're using; all they need to do is look at the headers of your message:
MIME-Version: 1.0
Content-Type: text/plain;
charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Mailer: Microsoft Office Outlook 11
Thread-Index: AcZBxMbp3pklllG2TIyRhkP5Ax1cTQ==
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180
To change this, all you need to do is;
C:\Program Files\Microsoft Office\OFFICE11\OUTLMIME.DLL
MIME-Version: 1.0
I'll see how to change the X-MimeOLE: header next.
Content-Type: text/plain;
charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Mailer: Micro$oft Arse Mailer
Thread-Index: AcZBxMbp3pklllG2TIyRhkP5Ax1cTQ==
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180