Can't receive a mail outside my organization in my firm (outside our local network) -on outlook application !

Hi,
A new colleague of mine having trouble receiving an email outside our organization (local network), she receives regularly mail (from us colleagues), she can send it..but when I send her from my private mail (for example) a message. Nothing happens. Just to be clear, this happens on an outlook application, online when she is using it. Everything is fine. Maybe some simple setup is a problem? We are using 365 Outlook!