A common misunderstanding when prioritizing a message in Outlook is that it also will affect message delivery. In other words, if you mark an email as high priority it would automatically be delivered faster on a busy Exchange Server compared to a normal email message. This isn’t true. But it could be. Here’s how.
If you want this kind of behaviour you need to enable Priority Queuing in Exchange. There are lots of configuration possibillities but the one magic switch is
<add key=”PriorityQueuingEnabled” value=”true” />
inside of the EdgeTransport.exe.config file on your Mailbox Role Server in Exchange 2013. This feature is also available in Exchange 2010 but then you need to edit the file on your Hub Transport Servers instead. Actually, this feature was introduced in Exchange 2007 SP1.