With email now in house on an Exchange server, you should reconfigure Outlook to use the Microsoft Exchange service instead of Interenet mail Service (POP3). Once you isntall Microsoft Exchange service, it will ask you for the user mailbox and server name. As long as the boss is logged into the network, he should be able to get into his email box without a problem.
If you still want to keep pop3, then under the server name, make sure the Exchange server name is located in your LMHOST files,or hopefully you are running a WINS server on the network to identify the netbios name of that computer. (I.e. WINS lets you know that "mail" is your exchange server at 192.168.72.xxx, for example).
If all else fails, in the server name, put in the IP address of your exchange server for both incoming and outgoing and see if that works.