This is not a PHP-Nuke error. PHP-Nuke uses PHP's mail() function and this, in turn, uses the mail server that is found on the web server. If that mail server is configured incorrectly, you will get problems with mail. The best way to solve them is to talk to your ISP and try to find out which configuration is used for your case. We have seen cases of ISPs that would configure the mail server to deliver mail only to local domains, thus making communication between PHP-Nuke and users of other domains impossible (see E-mail problems, my nuke only sends mail to my domain).
Now, what do you do after you have fixed your mail server? Those e-mails with the activation links have already been sent once, and since they never reached their destinations... You are losing users because you cannot resend them!
If you ever needed to resend an activation email, or just wanted to delete one that was awaiting activation, then you will appreciate the Resend module. It will help you manage the temp_users table.
If you want users to be registered directly, without confirmation mails, see Section 18.6.5. If you want to disable registration, see Section 18.6.4. If you want to approve every user who applied for registration, you can use the Approve Membership module (see Section 8.3.4 and Authorize accounts).