Unable to successfully login with UPN

Hi there,

I have a rather interesting problem I'd like to resolve.

I'm working in my test environment with AD in a Windows 2003/2012 forest.  The forest domain is: Vitaluk.com

I use email addresses of: firstname.surname@piasuk.com in this domain and email has been configured on exchange 2013 in a hybrid setup with Office 365

My test users can all log in fine on premises when using the formatVITAL\username  but none of the test user's I've set up with a UPN ofusername@piasuk.com in Domains and Trusts are logging in.  I've only managed to get a few working.  

I get the message:  unknown username or bad password when I use the UPN to login

Ideally I'd like everyone to be logging in using the UPN as this is the same username for Office 365 access.

Note: I've tested UPN usernames on the same laptop, and this is correctly syncing time, so Kerberos time isn't the issue.

Any ideas?

thanks,

Mike