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Unlike other mailers, recvmail doesn't require any root privileges. None. Zero.
The entire program runs inside of a chroot(2) jail.
Mailservers are notoriously complex beasts that can be hard to set up and configure. Recvmail does not use a configuration file, and doesn't require any command line options. It is the easiest MTA you will ever use.
Recvmail can receive mail, but not send it. Mail flows into the system from external hosts and is stored in one or more mailboxes. It is never forwarded, redirected, or bounced. This makes the design of the server much simpler, and less error-prone.
In the Recvmail system, mail is delivered directly to the destination mailbox and is not stored in a "queue". Most other programs store messages in a temporary directory, which increases the overall complexity of the system and decreases reliability.
When a sender specifys multiple mailboxes to receive a single message, Recvmail stores a single copy of the message on the filesystem and creates multiple hard links. This is more efficient than creating a unique copy of the message for every recipient.
Recvmail uses streaming I/O instead of buffering the entire message. This means that after each line of input is received from the sender, it is written to disk. This approach minimizes the amount of memory required to process each message, and makes good use the operating systems buffer cache.
Recvmail uses libevent to handle all client connections inside a single process. This is more efficient than using a dedicated thread for each client connection.
If there are errors, they are reported to the sender during the SMTP conversation. Traditionally, the sender would hang up, and errors would be returned using a "bounce message". For various reasons, Recvmail does not exhibit this behaviour.
As soon as possible, Recvmail permanently and completely drops root privileges, and the entire process is restricted to a chroot(2) jail. Most other software is designed to have one privileged component that interfaces with other non-privileged components. Since Recvmail uses virtual accounts, there is no need for it to have access to the system accounts.
Instead of providing many options and features, Recvmail tries to choose the best way to do things and then does it. For example, it uses Maildirs instead of mbox; it uses virtual accounts instead of system accounts.