This workflow resides in a document library that is not trusted

There's a first time for everything, and after extensive testing on an list requiring approval from within Outlook, one of our testers got the following message: 'This workflow resides in a document library that is not trusted'. We tried all the obvious things, adding the site to the 'Trusted Sites' section in IE, checking browser settings/ versions etc. and just couldn't get the bottom of it. The fact that the tester was working on a pretty non standard PC build for our organisation ( Windows 8, Office 2013, IE10) turned out to be a little bit of a red herring. The solution that got us there in the end was to out the full URL into trusted sites ( i.e: http://sitename.domain.com) instead of just that site URL (i.e. http://sitename.com) .

Various forums had unresolved threads on this topic, so thought it would be worth putting out there. You just never know.