Outlook 2007 Calendar invites from external domains ie NOT from within Zoho.

Outlook 2007 Calendar invites from external domains ie NOT from within Zoho.

I use POP with my Outlook 2007 email client on my home machine, Just recently I have been getting a problem with Calendar invites sent from my work Outlook calendar to my home calendar, all I get is a text based email and no option to accept, or decline the calendar invite?
 
This has only started happening recently , no changes have been made to my machine, and my wife is also having the same problem on her machine using the same Outlook client/
 
If I send between my Zoho accounts it works fine but seems to be a problem with extrernal domains.
Any ideas?
 
thanks