Tips when upgrading from Exchange 2003 to Exchange 2010:

- If you have users with Outlook 2003, be aware of the following:

o Outlook connection issues with Exchange 2010 mailboxes because of the RPC encryption requirement https://support.microsoft.com/kb/2006508

o E-mail messages take a long time to send and receive when you use an Exchange 2010 mailbox https://support.microsoft.com/kb/2009942 

- You need to verify your e-mail address polices and clean the ones that you don’t want any more because If you have any Exchange 2003 recipient policies that have not been applied, making any change to an Exchange 2003 mailbox by using Exchange 2010 tools will force the recipient policies to be re-evaluated and applied to the mailbox. Before you make changes to an Exchange 2003 mailbox, make sure that you want to apply all of the existing recipient policies. If you have an existing recipient policy that you do not want to apply, clear the ‘Automatically update e-mail address based on e-mail address policy’ check box in Active Directory Users and Computers.

- Notify your users early enough about the move so they can restore their deleted items before you move their mailboxes.