microsoft exchange server issue.

Were they in a BCC list?  
Check the SMTP headers and or Message Tracking logs to see how the email was delivered.

If user/mailbox owner has not sent that emails,

1> Somebody else might have sent the email from that mailbox (sharing passwords)

2> A few viruses can do this. They pickup any address from the address book and sends emails out through email gateway configured on system (applicable to Outlook express, Outlook - applications where SMTP address is mentioned)
 
To verify both options, you need to check Message Headers!

how to check the message headers in the exchange ..please help..BCC is not used.
SMTP headers . how to check??

Please refer image result search http://images.google.co.in/images?hl=en&q=internet+header&oq=&um=1&ie=UTF-8&ei=l6KDS4q4EYvW7AOR1d1G&sa=X&oi=image_result_group&ct=title&resnum=4&ved=0CBgQsAQwAw&safe=active

If email is delivered within Exchange Orgnization, right click on that email in Outlook; select options and check "Internet Header Section". Please read it from bottom to top.

i am unable to check the issue, hence i dont want to comment. it was offcourse helpful with the ideas.