Mixed mode assembly is built against version 'v1.1.4.122' of the runtime ... while using the faxman component under Visual Studio 2010 and .NET 4.0 there is a work-around.
Open your app config. Your startup segment should look roughly like the one below.
Change the opening startup tag to contain the "useLegacyV2Runtime..." element.
Adding this line should get you building and running in no time until Faxman comes out with a newer implementation.
I hope this can get you working again.
David Cross