What is a good third-party SMTP software for custom logging?

I am currently using IIS 6 SMTP to send out some of our mail and I unfortunately amn't finding the logging performed by it to be verbose enough - nor am I able to customise it to the level I would find verbose enough.

I'm wondering if anyone can recommend a piece of third-party SMTP software which would allow for customisability of the logging down to the point where I can inject in my own columns or perform regular expressions on the outgoing message data or headers.

How's your Ruby?

I only ask, because I'd probably solve this problem by using Logstash, and writing a custom input plugin to accept SMTP data. Or use some kind of external filter to make SMTP data be adequately represented in one of the existing accepted input formats.

Edit: By external filter, I'm hinting at something written in a suitable scripting language (Perl, Python, etc) that will act as a SMTP client (or server, depending on your POV), connect to the SMTP server, grab the message, and then make it into a format that Logstash accepts natively, be it AMQP (my preference for this), or raw TCP.

Edit 2: I've just found a project for converting SMTP to AMQP using a rabbitMQ plugin. A bit of a long way around, potentially, but still quite a neat idea. You'd set up a RabbitMQ server with this plugin, that would then convert SMTP messages to AMQP, which Logstash would then consume and analyse.

It's a bit of a niche problem, which is why you'd probably do better writing your own, or writing a plugin for an existing something.