Jetpack has a subscribe by email feature. If the wanna be subscriber does NOT go to his mailbox and confirm its receipt, Jetpack still puts the wanna bee subscriber as a subscriber.
This is HORRIBLE. Now I have a list of SPAMMERS and not subscribers. The Spammer can put anyone's email address or a fake email and still have subscriber status on my website.
Free is good for this program. But Free to receive SPAM is BAD. Very bad.