Ask HN: sendmail vs mailto: links for viral content sharing?

I have a widget that has share-on-digg and email-to-your-friends type functionality. I've implemented the emailing in what seems to be the standard fashion - a minimal dialog allowing entry of the to/from details, with the email then being sent using sendmail from my server. However there seem to be lots of issues with scalable email sending - host throttling, getting caught in spam filters etc. I'm wondering why I shouldn't change to using a browser mailto: link. I can still fill in the needed email content but the sending is then done from the users local machine and email mechanism.

Having each user do their own sending seems like a more scalable and robust solution to me. But it doesn't seem to be standard practice with similar apps. Any thoughts on why? Are there a significant percentage of folks who don't have an email client set up correctly for mailto: to work?