I've a perl mailer script (using Sendmail) to email a notification to a specified email address after a form is filled out.

I've tried this site's Forgot Your Password mailer and the email reached my Yahoo Inbox.

Are you talking about the same server that is sending the emails? First you talk of "I have..." and after that "I've tried this site's", which makes me think the second one is not the same as the first.

Your sending MTA's IP address might be listed in some RBL. You can test that at sites as http://rbls.org/ or http://www.declude.com/junkmail/support/ip4r.htm.