This issue has been solved with the help of Ironport technical support team. Below is the description of the issue
In fact it is another bug (#84518) which matches the problem our problem. This bug describes an issue where Outlook does not display message body when body and footer have different charsets.Charset used for message body is "iso-8859-1" and charset used for Disclaimer is "UTF-8". That is most likely causing the issue on the recipient side.
As this is a bug they are still in state 'work in progress' by development,they as ask to try changing the encoding of the disclaimer to iso-8859-1 and to send a few test messages to see if this would be a workaround will works fine or not.We create a new disclaimer for testing by selecting"Western European/Latin-1 (ISO 8859-1)" as encoding and some text mails were send amd it worked fine, and this work round was a sucessful one.