A PostgreSQL server is required. It is recommended to use at least a 8.x version.
For the mail-database exchanger: a Unix system that is already capable of delivering incoming mail to a local account, and sending outgoing mail by way of a local mail agent. Here are some programs that are most commonly involved in such a setup:
The Qt library (version 4.2 at least) and the PostgreSQL client libraries are required.
For popular flavors of Unix such as Linux, these libraries are generally part of the distribution, otherwise they can be built from source.
For Windows, they're included for convenience in compiled form in the manitou installer archive. See the download section.