Sharepoint 2013 - Share site invitation email does not work?

Hi,

Recently upgraded to SharePoint 2013, but the share website / invitation feature does not work.  From the server I can telnet over port 25.  The AV is not blocking, nor the Firewall.  The IP is allowed over HubRelay for Exchange.

I've been through this article:  http://technet.microsoft.com/en-gb/library/cc263462.aspx#section2

Any ideas?  Thanks, Matt.