I would like to add a spam catching module. I'm aware of the CAPTCHA option, but I don't want to use it because many users find such "tests" to be confusing and frustrating.
I have a simple PHP script I developed that interacts with data from spam-ip.com's list of known IP addresses for spammers and quietly ignores any form posts sent from a known spammer's IP. I'd like to integrate it if possible, perhaps via a separate plug-in, but I'm not sure how to tie it into your form processing functionality.
My script simply detects the user's IP address, checks that IP against a regularly updated list from spam-ip.com that I maintain on another web site, then if it doesn't find a match in the updated list, it integrates the IP address into the mail message and turns that IP into a link to another web page I maintain on another site. This other page gives me exclusive access (based on my IP address) so I can use a special form I've set up there to report a new spammer that got through.
My system is about 95% effective in blocking spammers, and it gets more effective over time as I report more spammers to spam-ip.com.
I believe there is a similar plugin for comment forms called Spam Free WordPress, although that author's plugin is considerably more comprehensive than mine. If there was a way to link that plugin to your plugin, it would be the ultimate solution.