If you are an Outlook 2013 user then you may be aware of this issue related to how Outlook 2013 displays custom form icons: http://www.symantec.com/docs/TECH213877 and http://www.symantec.com/docs/TECH211221
Happily MS have released a fix for Outlook 2013 to resolve this display problem. You can download the MS update here: http://support.microsoft.com/kb/2878323. This fix is also part of the Office 2013 cumulative update for April 2014 so many of your clients will be patched automatically.
