The log files would be the place where all of the information you are after is stored. There are several log file analyzers available, but your choice will be influenced by what mail server you are running.
To get you started, a popular one is pflogsumm. I also came across one called sawmill (sawmill.net), which claims to analyze 843 different email program formats.
If you are looking to "filter" you may want to consider setting up some form of rules list that will deny / allow from and to certain recipients.
Also, one caveat: you mention employees and as the employer you (probably (depends on your local laws)) have the right to be BCC'd and examine all of the email. Unless you have cause to suspect a particular problem, with a particular individual, you may want to think carefully about doing so. Your employees would likely view that as a violation or at least see it as a lack of trust and respect which could have negative consequences. Examining the logs for traffic patterns is one thing, reading all of the correspondence is another.