I was testing this in my lab, and of course used the default Administrator account. I kept getting the error of :
Warning:An item has failed property validation and will be ignored.
IdentifierName: Subject of message in inbox
Error details:Required property is empty.
I had created the correct impersonalisation permissions on the Client Access Servers and also added the following line into the EWS web.config file to allow mail to be imported over 3mb:
However I was still getting this error.
I then remembered reading an article on TechNet about the Administrator account not being able to log on via IMAP4 or POP3, as it is hard coded to be blocked by default.
I then created a 'migration' account user in AD, and gave that using Exchange Administrator permissions and the relevant impersonalisation permissions, and was able to succesfully migrate Internet email into Exchange using both POP3 and IMAP4.
You can download the Microsoft Transport Suite of tools here:
MVP - Exchange Server