When you convert on-premises mailboxes to mail-enabled users (MEUs), the proxy addresses and other information from the Office 365 mailboxes are copied to the MEUs, which reside in Active Directory in your on-premises organization. These MEU properties enable the Directory Synchronization tool, to match each MEU with its corresponding cloud mailbox.
Using the steps provided here I was able to export a list of only user mailboxes and leaving out other mailbox types
Run the following PowerShell command:
Get-Mailbox -ResultSize Unlimited -RecipientTypeDetails UserMailbox | Select PrimarySmtpAddress | Export-Csv -Path .\migration_users.csv -NoTypeInformation
The following full steps of what to do next can be found here: http://community.office365.com/en-us/w/exchange/835.cutover-exchange-migration-and-single-sign-on.aspx