Looking for recommendation for a light-weight, SMTP server, with basic capabilities (will be run on port#25, with authentication but clear-channel), and it's use shall be limited to a particular (or a small set of) domain(s), i.e. mail to/from same set of domain(s) for authenticated members (both to/from fields) only shall be accepted, and of course, with additional spam filtering (bayesian possibly).
Target platform is Ubuntu Server (10.04.01) / CentOS 5.5.
TIA, ~i++