Bounced and undeliverable emails make you look bad in the eyes of your provider.
And if you cannot reach your list, you can't sell.
WP Inbox Monster takes care of this and also comes with MRR license, means you can sell it.
You can choose that people cannot sign up to your list with fake email or even exclude freemailers like yahoo.
I am out of space, google for more info, pls, or ask BEFORE you buy.