norelaysmtpd is a lightweight mail server which sole purpose is to receive incoming messages and deliver it to local users.
 Its main "feature" (or non-feature, really) is to be completely unable to relay messages to adjacent servers: if an incoming message is 
 not addressed to a local mailbox, smtpd will reject it. Another convenient feature is its ease of configuration: creating mailboxes or aliases 
 boils down to simple filesystem operations (mkdir, symbolic links, etc.). It can handle incoming e-mail traffic for an unlimited number 
 of domains (i.e. act as a MX for these domains) without the need of any configuration (other than creation of users' mailboxes).