If Pobox handles your domain registration, then a WHOIS of your domain will show an address at domain.pobox.com as the administrative email for your domain.
On January 1, 2014, ICANN added a requirement that all domain owners be verified under a variety of circumstances. Failure to verify your contact information results in the suspension of your domain, whether it is paid for or not.
Because the ability to change your email address is one of the features of a Pobox account, having another static address that you needed to maintain seemed unnecessary. Domains registered through us use a WHOIS email address <yourdomain>@domain.pobox.com. That address forwards to the administrative address for your account. This avoids the need to keep your WHOIS information up-to-date, and lets us monitor for messages from our domain provider that could result in the suspension of your domain's services (which are otherwise outside our ability to fix.)