Not able to send meeting request to MS outlook users

Hi,<o:p></o:p>

One of our
customer is using MS office 2010 & office 2007 to access email &
calendars. when they send meeting request from our outlook client to some
other organization it is getting submitted in some txt format. We have tested
it in outlook versions. <o:p></o:p>

But if the
meeting request for some Gmail or non MS outlook user then user is getting the
request with correct information. <o:p></o:p>

Email Setup
Detail: Server - Linux based sendmail & Client: MSoutlook 2010, 2007<o:p></o:p>

 <o:p></o:p>

Regards<o:p></o:p>

Sanjay<o:p></o:p>