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.
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).