Fresh install of WordPress 3.8 MU + WPMU DEV Dasboard + Akismet + Jetpack + Update Services + Fourteen colors + Positive SSL. Everything works.
I installed the Signup Code and activation emails stops from sending.
I installed Unconfirmed plugin to see how many people is waiting for the emails. I tried to resend activation emails couple of times, and no use. And after deleting the Signup code later new members did receive emails. And I tried to resend emails to unconfirmed users once again, and it worked.
At first I thought SMTP wasn’t configured properly, but now I know that this is the plugins problem.