Thank you for your reply, felgall. I believe the suggestion that you put up there is more onto the form / front-end part of the website which I believe my site have already adopted some of your suggested approaches.
In my case however, the issue that I am dealing with is more of spelling error email addresses, typos, or random emails with valid email format which the users may either may or may not deliberately did it (eg. firstname.lastname@example.org, email@example.com, etc).
I came across this one article here: http://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-address-exists-without-sending-an-email/ on verifying email addresses programmatically using PHP. After many hours trying the suggestion listed there, I've found out that this approach has a weakness also whereby the response is entirely depends on the mail server host itself whether they support message relaying or not.
My question here is, is there any other way to do an email address verification apart from using the approach as detailed on the url above? or what is the other way around to prevent this kind of non-existence/random email issues from getting through?