This paper introduces an MPI test suite to thoroughly test the correctness of an MPI-implementation. Scientific applications require stable tools and libraries for portable and efficient programming, e. g. when depending on the single-sided communication on multiple platforms. This test suite was originally used to check the correct transmission of data in the PACX-MPI implementation, but has been mainly used and extended to test the development of the new Open MPI implementation. The tool has been designed to be easily extendible in order to integrate new tests using the underlying functionality