https://townhall.com/tipsheet/leahbarkoukis/2016/10/27/obamas-blackberry-offers-more-proof-white-house-knew-about-clintons-email-n2237741

President Obama’s high-security BlackBerry used a special process known as “whitelisting” that only allowed it to take calls and messages from pre-approved contacts, two former senior intelligence officials with knowledge of the set-up told Fox News – pointing to the detail as further proof the White House knew Hillary Clinton’s private account was used for government business. 

As the administration now acknowledges, Obama and Clinton emailed each other while she was helming the State Department. If received on his BlackBerry, the “whitelisting” safeguard means Clinton and other contacts would have had to be approved as secure for data transmission – covering everything from emails to texts to phone calls. The Obama BlackBerry would have also been configured to accept the communications.