MS Exchange 2003 is no longer supported and Outlook 2013 in not compatible with it. Client will need to downgrade their Outlook version if possible or purchase Outlook 2010. (Volume License agreements offer downgrade rights) Alternatively, you can set up their Exchange account as IMAP. Also, if clients use Comcast Business Class service they can install Outlook 2007 for free typically up to 10 licenses.
Ultimately the client should seriously consider upgrading their server as 2003 is now at it’s end of life cycle.