Hi, I have three Exchange 2003 Servers. One of them is back-end for few databases and SMTP connection. The second Exchange 2003 is back-end for another few databases. The third Exchange 2003 is front-end for OWA. I have setup two Windows 2008 Servers. One will be for mailbox role and other will be CAL and Hub Role. I have installed all the prerequisite, domain preps, and all. My question is to install Exchange 2010, should I install for mailbox role first or the CAL & Hub Role? Or it really doesn't matter? Also once I implement the Exchange 2010, I will move my mailboxes to Exchange 2010. I was also plan to deploy Exchange 2013 and then move mailboxes from Exchange 2010 to Exchange 2013. Can I do this while Exchange 2003 is still around till I move all the mailboxes from 2003 to 2010, then to 2013 till I decommission the 2003? Thanks in advance.

It is always recommended to install CAL -> Hub -> Mailbox order so in your case CAL and Hub role is recommended first and then mailbox. You need to first Uninstall all Exchange 2003 servers before you install Exchange 2013 server. It is prerequisites of Exchange 2013. Warning: You need to remove all instances of Exchange 2003 from your organization before you can upgrade to Exchange 2013. From: http://technet.microsoft.com/en-us/library/jj898583(v=exchg.150).aspx