mailx not sending to Outlook distribution list?

So I have a Solaris 11 server with mailx. I am trying to send emails to distribution lists in the company's Outlook (which is not running on this server, it runs on Microsoft's cloud). I am able to send emails to individuals without trouble. Then I am able to send emails to SOME distribution lists, but not others. A coworker of mine who has been here a long time told me there was "some setting in Outlook" that allows mailx to send to distribution lists. Other people have told me I need to set up an alias file (I've checked the alias file on the server, it is empty, so that does not explain why some distribution lists are working but not others). By "not working" I mean just that- the emails don't send. They appear in /var/log/syslog without errors. But the emails are never received. I'm at a loss as to what to do. Any help at all is appreciated.