If you are putting together a tagbase yourself you run the risk of infecting the content with your own personal perspective of what should and should not be tagged, whereas the intention of tags is for them to act as the users' own interpretation of what is meaningful to them.
Depending on the role of a person within the company, different people will be interested in different sets of tags - whether for marketing, technical, hr, sales, management, etc. My bet is that if you asked people to assign tags to the same bulk of prior mail you will get different sets of tags from each role, let alone the differential between the actual incoming mail that is assigned to each person.
I think it will be very hard to correctly initialize a valid static tagbase, without allowing it to evolve naturally. You might want want to consider a data-collection phase whereby for some period of time, people at the company each determine the tags that they would use on their incoming mail and then feed this back into your process, thus getting real users input on real data to instantiate the tagbase for the implementation phase. You might learn something interesting in the process too about what users actually understand about tags and how they use (or expect to use) them.