After a successful Exchange upgrade from 2010 to 2013, there are 4 users that have emails delayed both internally and externally. When someone sends the 4 users an email at 11am for example, that user does not receive the email until hours later and sometimes the next day.
There is no 'delayed' message to the sender.
Microsoft Windows Server 2012 R2 /Microsoft Exchange Server 2013 SP1
There are no errors on the server or the users workstation indicating that there is a problem. What else should I be looking at?