3
You have to be very careful on who do you activate.
Some of the unactivated users could be SPAM users. If you activate them, you'll be opening a door to SPAM. I always have a bunch of unactivated users. Some of them received the activation mail, that went directly to the SPAM folder. Some others are simply SPAM users. I slowed this process down using DuGris' CAPTCHA.
Xoops 2.2.3 has a "resend activation mail" page, called "activate.php". XOOPS 2.0.16 hasn't, don't know why. Just get XOOPS 2.2.3 and pick up this file. It won't work at first try, but it can be easily adapted. Dabug console is your friend.