They just added this new rule in their TOS, so be sure to take a look.

QUOTE
This has been added to the terms today:

This is a program only for active users. If you do not click on any paid emails or activity check emails for 40 days your account will be terminated without a warning and you will forfeit all prior earnings. If you are going away for a timeperiod longer that 40 days you need to send an email to webmaster@phantom-mail.com before you leave to prevent that your account is terminated.