I wouldn't touch this without first giving it a very thorough security audit. Zed is right that most MTAs in use today are Byzantine horrors, but he misses out on why they were written that way: security. A mail server needs to bind to a privileged port, aggregate every user's mail into a single outgoing queue (which must be stored on disk for reliability), setuid into arbitrary users for delivery, and sometimes execute arbitrary commands given by those users (procmail). This is not an easy thing to get right. The reason for the strange design of Postfix, qmail, et. al. is to allow them to implement privilege separation wherever they possibly can. The thought of throwing all that away, and substituting a lightly-tested, monolithic program written in a dynamic language that permits monkey-patching at runtime and doesn't have Perl's taint checking support, makes me nervous to say the least.

Mail servers, web servers, C compilers, libcs, and crypto libraries all seem to share the common property that their basic functionality seems enticingly easy to implement, inducing the naive to look at the mainstream implementations and think, "Look how stupidly bloated these are! I can do way better", and go on to produce nice elegant, minimalistic implementations that immediately fall over when put into production.