If this issue has lasted four years and has yet to be rectified, why continue to deal with Link for that client?  There has to be a better way to transfer for information between you and the client than to continue to deal with Link for that one client.  Don't get me wrong, after four years you would think the issue would be taken care of and access for the client could be established.  

I'm new to ProSeries this year and plan to use Link for my clients.  I've tested Link using my personal email and it seems to be a straight forward process.  Hopefully my clients agree.