The aim of this work was to analyze and investigate how reliable and redundant mail servers must be configured in today?s modern IT environment. High availability is a topic which is more and more relevant in today?s world of operational IT-systems. Therefore, both storage management and operating system, including key-applications, need to be configured in an appropriate way. In this work, MS Exchange 2003 and IBM/Lotus Domino 6.5.2 mail systems were installed on a Windows Enterprise Server 2003 operating system running on a redundant HP ProLiant server environment. To meet the requirements of both reliability and redundancy, a SAN and hardware-clustering were used and implemented. Four HP ProLiant DL360 servers and one HP StorageWorks MSA1000 with an integrated fibre-optic switch were provided for the realisation. On the basis of the chosen configurations, several failure scenarios were tested and the results analyzed and interpreted. A main focus was set on the ?mailbox restore? and ?mailbox move? functions on both mail. The decision which mail server should be used in a company is highly dependent on the business requirements in a modern corporate environment as well as on the range of features, availability needs and the direct responsibility of the employees. Exchange?s functionality pales in comparison to Domino, but offers lower operational and licence costs. Domino?s innate availability is lower than Exchange?s; nevertheless its winning advantages are the wide range of administrative and operational features as well as the cross platform compatibility to other operating systems. Where Groupware and Workflow should be used and optimised, a company should definitely decide to implement a IBM/Lotus Domino mail system. The test results show major differences in availability and usability, which are highly dependent on the administrator?s know-how - and not only on functional aspects.