First, I would like to explain what a safelist or mailer is. They are very much like a traffic exchange in that people are sharing websites with each other. The main difference is that an email comes before viewing the site. Members receive emails from other members and a link inside them gives credit for viewing the site within the email. The credits are then used to send your emails out with your website, ad, or offer. With this in mind, remember that a lot of emails are going to be coming to your inbox, especially if your a member of multiple mailers. You might want to create an email address specifically for mailers.
Here are the top mailers that we have selected in which we have had results that we can verify personally. They are ranked in this image: Top Mailers