sun818 - 5:28 pm on Jan 20, 2012 (gmt 0)
I don't have any A/B data to prove this, but I tend to think web forms that don't require an e-mail client will increase e-mails due to the lower barrier for communication.
For Contact Us forms, you need a verify e-mail follow-up. You end up getting a lot of false e-mails either as pranks or bad spellers. firstname.lastname@example.org would require the sender to have an e-mail client, or a browser add-on that sends them to their web based e-mail account. This significantly removes the chances of a bad FROM e-mail, but not completely. I've been known to incorrectly type in my FROM address sometimes on my e-mail client.