Email Solution

We are using IntraPoP for Email in our company this s/w is developed by indian s/w company on NT platform using ASP. In this s/w there is some bugs so that we want to use MS Exchage server on lease line but we have limited budget We can purchase only 30 user licence but we have 100 users. Is there is any solution ? Like in MS Outlook can we make profiles in Exchage server or can we create a user and then subuser. Please guide us as this is urgently required.