It sounds to me like the problem here is basically that some people are using the commenting system, to post their comments, and are entering a "mumbo jumbo" (false) mail account... seeing as they need not be registered on your site to post comments, they can type in anything in that field, as long as they pass the CAPTCHA system.
But they must be REAL people for it to work.
Once a person has posted a comment, then it will (of course) be notified to the others that have posted their real mail address, and checked the Notify box. The fact that the mail sending came from a comment that has been written by someone with a mumbo jumbo mail address is not a security issue. -it is still a real person.
I mean, (as long as I've understood this correct), the system cannot determin whether an entry is a good one, or an idiot.
So people will be notified in their INBOX regardless of what kind of posting is made.
My site allows ONLY registered users to post comments... and even then some people just love to post weird comments. Ok, I chuckle a bit etc, but I haven't seen a single spam or some indication that it is a security issue at work.
Or do you mean that people MUST pass an E-mail verification first, prior to posting?
And are somehow getting around this?