Exchange 2010 purging email from all mailboxes

I am trying to purge ALL email from our Exchange 2010 environments that is older than the year 2012. I created a Retention Policy Tag for Inbox, Sent and Deleted Items with the Age Limit of the number of days back I wanted to start my purge. I then created a Retention Policy, added in the new Retention Policy Tags and assigned all users in the organization. After the maintenance window, some emails were purged but not all. It seems like the emails being purged are doing so based on the email modified date instead of the received date. Also, I don't see a way to verify the emails were purged. I work for a local government so verification is very important to comply with record retention policies. Is this the best way to purge emails or is there a better way?