I had to add a second account here, with another e-mail provider, since my registration from @inbox.com wasn't successful.   
Also, if I try sending e-mail from my HelioHost account (say from cPanel/webmail, with a parked domain like yyy.co.cc) to an account on inbox.com I always receive a bounce of my message, with the errors: 
"lxxx@inbox.com 
    SMTP error from remote mail server after MAIL FROM:<xxx@yyy.co.cc> SIZE=1836: 
    host inc.inbox.com [64.135.83.90]: 551 Server access WH97.inbox.com forbidden by invalid RDNS record of your mail server (65.19.143.2)."   
From the above I deducted that all the messages requesting validation from this forum, to my "lxxx@inbox.com", have a bounce back... Is it so? 
Are you aware of this kind of problem?   
~ Luca