Most of my emails aren't being received

I just made a new account on outlook but about 3/4 of my emails aren't being properly sent; as in, they show up in the "Sent' box, but the recipients are telling me that they have received nothing - however, some recipients do receive the emails for some reason. One common thread is that literally none of my replies go through (only some of my new emails do). I've had difficulty contacting Outlook directly for help.