smtpd is a lightweight mail server whose sole purpose is to receive incoming messages and deliver them 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 automatic grey-listing of clients that allows you to efficiently reject spammers based on their behaviour. It can handle incoming email traffic for an unlimited number of domains (i.e. act as a MX for these domains) without the need for any configuration (other than creation of users' mailboxes).
popdns is a small daemon that aims at simplifying the use of roaming machines by allowing them to dynamically update their DNS entry using (nearly) any POP3-capable email tool. After having configured a new email source (with the POP3 protocol), users are authenticated using their system password and popdns automatically updates their DNS entry with the IP address of the connecting machine.