I would like to see Lync 2013 updated to have the ability to work properly in an environment that has both Exchange 2007 and 2013 running in a coexistence scenario. Currently if pointed to Exch 2013 but your mailbox is not migrated yet, Exch 2013 does a fine job at being a proxy and not causing an outage to the end user. However Lync constantly prompts with "Exchange needs your credentials". No matter what you enter here, it is not accepted. I guess the workaround is to migrate the mailbox to 2013. However in a large company you do not have time to IMMEDIATELY migrate every single user's mailbox in one weekend (if your lucky enough to have a business that is not open on weekends). There needs to be addressed in either Lync server and/or client. Perhaps a method in the Lync configuration can be implemented where Lync is informed of the coexistence scenario. This is still an issue even with Exchange 2013 CU6 installed. Tech Net forum users suggested opening this idea.