Hi Anshita,
I looked for a different way to embed Surveys directly into the Mail Body, without using an iFrame and came across a new solution which you could try:
Instead of using the iFrame functionality, you could also use the <object> functionality which would look something like this:
<p><object data="YOU SURVEY URL" type="text/html" style="height: 100%; width: 100%;">
<p>If nothing showed up except this text, <a href= "YOU SURVEY URL">click here to access the page directly</a></p>
</object></p>
Therefore is the the Survey embedded into the Mailform and if the Client does not support the <object> functionality, a Hyperlink is displayed with the corresponding Survey.
Please test this scenario in your System as I think this has more stability.
Regards,
Markus