Some people who register don't receive confirmation email

Okay, just like the title says. Some people who register in my site don't receive confirmation e-mail and I have to always go to my User&Groups admin>Inactive Users to check if there are inactive users and have to activate them. I'm busy cause I'm designing my homepage layout, so I can rarely remember to check on that.
Why don't some of the people who register don't receive confirmation emails? What will I do it so that everyone can receive emails?

Hi there wiper, what i would do is emal them off the site, from your own email, and tell them to add your sites address to their accepted emails list (how to do this differs depending on the email they use) because that will most likely be the reason they dont receive them